Background: Preschoolers' lifestyles have become physically inactive and sedentary, their eating habits unhealthy, and their sleep routines increasingly disturbed. Parental involvement appears to be crucial to combat unhealthy lifestyle of preschoolers. Because of recognized barriers of traditional face-to-face interventions (such as time commitment for parents), easy access and lower costs make electronic health (eHealth) interventions appealing. Previous studies that examined the effectiveness of parent-based eHealth on preschooler's (physical activity) PA, dietary behaviors (DB), and sleep have either emphasized on one variable or failed to balance the dosage of PA, diet, and sleep modules or consider the intervention sequence during the intervention period and there is an acknowledged gap in parent-based eHealth interventions which target preschoolers raised in Chinese cultural contexts.

Objective: This study aims to investigate the effectiveness of parent-based eHealth intervention on Chinese preschoolers' PA, DB, and sleep problems.

Methods: This two-arm parallel randomized controlled trial comprises a 12-week intervention with a 12-week follow-up. 206 parent-child dyads will be randomized to either eHealth intervention group or control group. Participants allocated to the eHealth intervention will receive 12 interactive modules on PA, DB, and sleep, with each module rot on a weekly basis to reduce the sequence effect on variable outcomes. The intervention is grounded upon Social Cognitive Theory and will be delivered through social media, where parents can obtain valid and updated educational information, social rapport, and interact with other group members and facilitators. Participants in the control group will receive weekly brochures on PA, DB, and sleep recommendations from the kindergarten teachers, but they will not receive any interactive components. Data will be collected at baseline, 3 months, and 6 months. The primary outcome will be the preschooler's physical activity. The secondary outcomes will be the preschooler's dietary behaviors, preschooler's sleep duration, and preschooler's sleep problems, parent's PA, parenting style, and parental feeding style.

Significance of this study: The parent-based eHealth intervention has potential to overcome the aforementioned barriers of face-to-face interventions, which will offer a novel approach for promoting healthy lifestyle of preschoolers. If found to be efficacious, the prevalence of unhealthy lifestyles among preschoolers may be alleviated at a low cost, which not only has a positive influence on the health of the individual and the well-being of the family but also reduces the financial pressure on society to treat diseases caused by poor lifestyle habits.

Inclusion Criteria:

1. Parents are beyond 21 years old and have 4-6-year-old children. Selection of this child's age range was based on two factors. First of all, the age ranges from 4 to 6 have been found to be a strong predictor of future health. Secondly, a significant barrier to healthy DB is food neophobia, commonly known as picky or fussy eating, a reluctance, fear, or refusal to have new foods that is common in young children aged from 4 to 7.5 years.
2. Parental commitment to participate in the overall 6-month intervention.
3. Parents have access to mobile technology.
4. Parents and children must be healthy (refer to a state of physical, mental, social, intellectual, and emotional well-being and the absence of disease) and can participate in normal PA.
5. Parents must reside with participating child for at least four days a week (for the children to be adequately exposed to the intervention that parents may implement).

Exclusion Criteria:

1. if they meet the WHO criteria for preschool age children in terms of PA (i.e., 180 minutes of in a variety of PA at any intensity, of which 60 minutes is MVPA), sedentary behaviors (i.e., no more than 1 hour at a time being sedentary such as sit in car, no more than 1 hour per day in screen time such as TV, phone, and computer), DB (such as daily salt intake should be less than 5g), and sleep time (i.e., over 10 hours good quality sleep including nap, with regular sleep and wake time).
2. they diagnosed with neurological or physically disable and children who have a parent suffering from a serious physical or psychological disease making the study too demanding for the family will be excluded from the study.
3. if they have special dietary requirements or allergies that would necessitate the tailoring intervention or that may be adversely affected by the intervention. Eligible participants will be required to provide informed consent by completing a written consent form after reading the participant information.