Table 1.

Descriptions and assessments of pre-specified criteria used to inform progression to a definitive trial

Description	Criterion met?	Assessment
1. > 75% of families upload steps at least 6 times in the first 3 months of pilot study.	Yes	• 86% of families uploaded steps > 6 times in first 3 months (mean ± SD = 11 ± 4 uploads).
2. Demonstrable feasibility of recruiting 20 families/month (accounting for increased staffing in future definitive trial) and retaining 75% of index children at 52-week follow up.	Partially	• The average recruitment rate was approximately ~ 7 ± 5 families/month (range = 2–15 families/month). • 88% of index children were retained at 52-week follow up.
3. Intervention optimisation feasible (identified adaptations are practical, affordable, acceptable).	Yes	• Focus groups revealed few suggested changes to the website; however, some families indicated a preference for the intervention to be delivered through a mobile phone app rather than a website. • Many families suggested receiving wrist-worn pedometers.
4. Evidence to suggest an adequately powered trial would require a feasible number of participants (N = 250 is considered logistically feasible and providing sufficient power).	Yes	• Post-hoc sample size calculations were performed and to provide 80% power to detect a difference of 10 mins in MVPA in index children, 27 index children/family are needed, using a standard deviation of 16.3 mins of MVPA and a pre-post correlation of 0.63. ○ With 3 arms: 27 index children * 4 people/family * 3 arms; N = 81 families (~ 324 total participants) ○ With 2 arms: 27 index children * 4 people/family * 2 arms; N = 54 families (~ 216 total participants) • Therefore, to conduct an adequately powered trial with a feasible number of participants it will have to be a 2-armed study.
5. Discontinuation of trial arm based on evidence of harm or limited acceptability/feasibility.	Yes	• There were no reports of harm, however, during the focus groups some pedometer only-armed families unknowingly indicated that they would have liked to receive several elements that were delivered to families in the family-arm (e.g., step challenges). This finding provides some evidence to suggest the pedometer only-arm could be discontinued in future.