Parent social support is measured using the abbreviated version of the Interpersonal Social Support Evaluation List (ISEL-12 (72)). The ISEL-12 has 12 items comprising three subscales: appraisal support, belonging support, and tangible support. We will use the total score for our main analysis (research question 4). Items are rated on a four-point scale ranging from 'Definitely false' (1) to 'Definitely true' (4). ISEL-12 total scores range from 12-48, and will be coded so that higher scores indicate more support. The ISEL-12 total score has good psychometric properties (Cohen et al., 1985). We translated the ISEL-12 to Dutch using the back-translation method. We also added two self-made items reflecting aspects of social support we were interested in in light of our intervention: having someone to look after your children (tangible support) and having someone to talk to about worries about your child (appraisal support). These will be analyzed descriptively.

Child overall psychological problems are measured using parent-report on the Strengths and Difficulties Questionnaire-Parent Report (SDQ-P; Goodman, 1997). The SDQ-P. The SDQ-P comprises five subscales with five items each: hyperactivity/attention deficit, emotional problems, behavioral problems, peer relationship problems, and prosocial behavior. We will use the 'general difficulties' score as our analysis outcome, which aggregates all subscales excluding prosocial behavior (thus comprising 20 items). We will perform additional analyses on the separate subscales. Items are scored on a three-point Likert scale ranging from 'Not true' (0) to 'Certainly true' (2). The 'general difficulties' score ranges from 0-40. We will use the Dutch translation of the SDQ-P (van Widenfelt et al., 2003).

Child PTSD symptoms are measured using parent-report on the Child and Adolescent Trauma Screener-Caregiver Report (CATS-C; Sachser et al., 2017; Dutch translation: Kooij & Lindauer, 2022). The CATS-C starts with 15 yes/no items about child exposure to DSM-5 A-criterion traumatic events. If the parent reports their child has been exposed to at least one of these events, the questionnaire continues with 16 (age 3-6) or 20 (age 7-17) items about PTSD symptoms. Data on PTSD symptoms are thus only collected if the child has experienced at least one A-criterion traumatic event. PTSD symptom items are rated on a four-point Likert scale ranging from 'Never' (0) to 'Almost always' (3). Interference with life domains is assessd with five yes/no items. We will analyze the total symptom score (range: 0-48 for age 3-6; 0-60 for age 7-17). We will perform additional analyses on the separate subscales. Scores on trauma exposure items will be reported descriptively.

Trauma exposure is measured at baseline using the Life Events Questionnaire for DSM-5 (LEC-5; Weathers et al., 2013). The LEC-5 comprises 17 items which describe DSM-5 A-criterion traumatic events (e.g., 'Fire or explosion'). Participants note for each event on a scale with 6 options whether they were directly or indirectly (i.e., as a witness, learning about it happening to someone close to them, or as part of their job) exposed to it. In case of multiple trauma exposures, the participant is asked to indicate which event bothers them the most. This event is called the 'index trauma' and will be used to determine trauma type in this study. The Dutch translation of the LEC-5 is used (Boeschoten et al., 2014).

General psychosocial functioning is measured as part of the ROM at Arkin using the Outcome Questionnaire-45 (OQ-45; Lambert et al., 1996). We will use the closest ROM measurement before baseline. The OQ-45 comprises 45 items, divided into three subscales: symptom distress (25 items, measures symptoms of depression, anxiety and substance abuse), interpersonal relations (11 items, measures relational functioning) and social role (9 items, measures functioning in school, work and/or leisure). Items are scored on a 5-point Likert scale ranging from 'Never' (0) to 'Always' (4). We will descriptively report the OQ-45 total score, which ranges from 0-180.

To provide information on the demographic background of the sample, participants complete a short demographic questionnaire (DQ) at baseline with items about themselves and their children.

For participants in the intervention condition, results on intervention adherence (completion of online modules; attendance of face-to-face sessions; and use of PTSD-specific content in the online library) will be reported descriptively. Furthermore, intervention group participants complete a short satisfaction questionnaire (SQ) at posttest. The SQ contains closed-ended and open-ended questions on aspects of the intervention, and invites participants to provide suggestions for improvement. Results will be reported descriptively in the publication of the current study and will be used for future improvement of the intervention.

PTSD symptoms are assessed in the EMA app using a short-form version of the PTSD Checklist for DSM-5 (PCL-5) with one item for each of the four symptom clusters (SF-PCL-5; Zuromski et al., 2019). Each item is rated for past-month frequency on a five-point Likert scale ranging from 'Not at all' (0) to 'Extremely' (4). The instruction of the SF-PCL-5 will be changed to fit with the EMA format. That is, participants are asked to report on their symptoms during the current part of the day (morning/afternoon/evening). Scores range from 0-16.

Sleep quality will only be assessed in the morning EMA assessments, using a single item ('How well did you sleep last night?') answered on a 10-point scale.

To assess the situational context, each EMA questionnaire starts with two situation items (Where are you; Who are you with), answered using multiple choice.

To assess current levels of distress, each EMA questionnaire contains a single item ('How much distress are you currently feeling?'), answered on a 10-point scale.