A culturally adapted, family-based behavioral lifestyle intervention delivered via WeChat for 16 weeks. Content is delivered through 3 prescheduled WeChat messages per week in health literacy-informed multimedia formats (e.g., short texts, infographics, and videos). The intervention targets healthy eating, physical activity, reduction of sedentary behavior, and parent-mediated behavior change strategies. It incorporates behavior change techniques including goal setting, self-monitoring, and positive reinforcement. Families receive automated prompts to complete weekly behavior-change tasks and standardized reminders.

Weekly coach-facilitated tele-group sessions delivered remotely via Tencent Meeting. Sessions last approximately 20 to 30 minutes and typically include 6 to 8 families. The sessions are delivered according to a structured manual by trained research assistants with master's degrees in psychology or related fields. Session content focuses on reinforcing key intervention messages, reviewing progress, problem-solving barriers, facilitating peer exchange, and setting weekly family goals.

This is a binary supportive secondary outcome, defined as the proportion of participants who achieve a reduction in BMI z-score of ≥0.20 from baseline to the end of the 16-week intervention, used to assess the clinical significance of the intervention effect. The BMI z-score is calculated using the exact same standardized protocol as the primary outcome. This outcome is reported as the responder rate (range: 0% to 100%), with a higher responder rate indicating a greater clinical benefit of the intervention on weight-related outcomes.

This metric reflects the change in body fat percentage from baseline to each follow-up time point, a direct and sensitive indicator of changes in children's body composition. It is assessed with a study-designated bioelectrical impedance analysis (BIA) device under standardized premeasurement conditions (including morning measurement after voiding, light clothing, and avoidance of vigorous activity prior to testing), with invalid measurements recorded as missing rather than imputed. A decrease in body fat percentage indicates the intervention reduces excess adiposity and improves body composition in children.

This metric reflects the change in daily moderate-to-vigorous physical activity duration (minutes per day) from baseline to each follow-up time point, a core indicator of children's health-enhancing physical activity levels. It is objectively measured via a research-grade triaxial accelerometer worn continuously for 7 consecutive days (including at least 2 weekend days) on the non-dominant wrist, with a protocol adapted for children with autism (including sensory-friendly adjustable fabric bands and desensitization guidance). Data are processed using the pediatric-validated Evenson algorithm. Increased duration indicates the intervention improves children's physical activity status.

This metric reflects the change in daily light-intensity physical activity duration (minutes per day) from baseline to each follow-up time point, a key component of children's daily energy expenditure. It is collected via the same 7-day triaxial accelerometer wearing protocol as MVPA, and classified using the same prespecified pediatric-validated algorithm to distinguish activity intensity. Increased duration indicates the intervention effectively reduces sedentary behavior in children.

This metric reflects the change in daily sedentary behavior duration (minutes per day) from baseline to each follow-up time point, an independent risk factor for childhood obesity and metabolic disorders. It is measured via the same 7-day triaxial accelerometer wearing protocol as other activity outcomes, with a prespecified algorithm used to distinguish waking sedentary behavior from different intensity activities. Decreased duration indicates the intervention improves children's daily activity patterns.

This metric reflects the change in average daily total step count from baseline to each follow-up time point, an intuitive and objective indicator of children's overall daily physical activity volume. It is continuously recorded via the same 7-day triaxial accelerometer wearing protocol as other activity outcomes, with the average daily value from valid wear periods used for statistical analysis. Increased step count indicates the intervention improves children's overall physical activity levels.

This metric reflects the change in average daily weekday recreational screen exposure duration (hours per day) from baseline to each follow-up time point, a core behavioral endpoint linked to sedentary behavior and obesity risk. It is assessed via a validated standardized electronic questionnaire completed by the primary caregiver, distinguishing recreational use from screen time required for school or therapy. Decreased duration indicates the intervention reduces sedentary behavior in children.

This set of metrics reflects changes in objective sleep characteristics from baseline to each follow-up time point, including total sleep time, sleep efficiency, sleep latency, and wake after sleep onset, which are closely linked to childhood obesity risk and overall health. They are derived from the same 7-day accelerometer monitoring period used for physical activity assessment, processed via a pediatric-validated sleep algorithm, with invalid data recorded as missing. Improved metrics indicate the intervention optimizes children's sleep quality.

This metric reflects the change in resting systolic and diastolic blood pressure from baseline to each follow-up time point, a core indicator of children's cardiometabolic risk. It is measured with a study-specified medical-grade automated electronic sphygmomanometer with an age-appropriate cuff following a standardized protocol (after 5 minutes of quiet rest, 3 sequential readings at 1-minute intervals), with the mean of the second and third readings used as the final value. Reduced blood pressure indicates the intervention improves children's cardiometabolic health.

This metric reflects the change in weekly frequency of sugar-sweetened beverage intake from baseline to each follow-up time point, a key dietary risk factor for childhood obesity. It is assessed via a validated semi-quantitative food frequency questionnaire completed by the primary caregiver. Decreased frequency indicates the intervention improves children's dietary patterns.

This metric reflects the change in the number of days per week with shared family meals from baseline to each follow-up time point, a core family routine indicator associated with reduced childhood obesity risk. It is assessed via a validated standardized electronic questionnaire completed by the primary caregiver. Increased frequency indicates the intervention establishes healthier family dietary routines.

This metric reflects the change in the number of days per week with regular breakfast consumption from baseline to each follow-up time point, a dietary behavior closely linked to weight regulation and metabolic health in children. It is assessed via a validated standardized electronic questionnaire completed by the primary caregiver. Increased frequency indicates the intervention improves children's daily dietary routines.

This metric assesses autism-specific mealtime behavioral problems, including food selectivity, eating stereotypies, and mealtime disruptive behaviors in children with autism, which are key factors influencing intervention adherence and dietary outcomes. It uses the validated Chinese version of the Brief Autism Mealtime Behavior Inventory (BAMBI). The total score of this scale ranges from a minimum of 18 to a maximum of 90, with higher total scores indicating more severe autism-specific mealtime behavioral challenges and worse clinical outcomes.

This self-reported metric assesses the primary caregiver's sleep quality and sleep disturbances over the past month, a key indicator of caregiver well-being and the family psychosocial environment. It uses the validated Chinese version of the PSQI, with total scores ranging from 0 to 21; higher scores indicate worse sleep quality and more severe sleep disturbances. This is a caregiver-specific outcome, not a child-level outcome.

This self-reported metric evaluates the primary caregiver's anxiety and depression symptoms over the past week, a core indicator of caregiver mental health status. It uses the validated Chinese version of the HADS, with total scores ranging from 0 to 42; higher scores indicate more severe anxiety and depression symptoms. This is a caregiver-specific outcome, not a child-level outcome.

This self-reported metric assesses the primary caregiver's parenting competence and self-efficacy, a key factor influencing intervention adherence and family implementation of behavior change strategies. It uses the validated Chinese version of the PSOC, with total scores ranging from 17 to 102; higher scores indicate better parenting competence and self-efficacy. This is a caregiver-specific outcome, not a child-level outcome.

This self-reported metric evaluates the primary caregiver's quality of life in the physical and psychological domains, reflecting the caregiver's overall health and well-being. It uses the validated Chinese version of the WHOQOL-BREF, with higher domain scores indicating better quality of life in the corresponding domain. This is a caregiver-specific outcome, not a child-level outcome.

This safety outcome assesses the incidence, nature, severity, seriousness, and study-relatedness of adverse events (AEs) and serious adverse events (SAEs) from enrollment to the end of follow-up. Data are collected via active multi-channel surveillance, including spontaneous caregiver reports, scheduled assessment contacts, and dedicated safety prompts embedded in the intervention platform. SAEs are defined in full compliance with Good Clinical Practice (GCP) standards. Intervention-specific AEs (including exercise-related injuries, medically unsafe weight loss, restrictive eating behaviors, and caregiver/child distress related to intervention participation) are prioritized for monitoring to comprehensively evaluate the safety of the intervention.