The family environment is an important social correlate of child physical activity (PA), with parent, and sibling PA levels positively correlated with child PA. Wearable activity trackers (wearables) incorporate numerous behaviour change techniques (feedback, self-monitoring, goal-setting) that has previously been identified as mechanisms of increasing child PA, in a family-setting. There is some evidence that wearables can increase steps, moderate-to-vigorous-intensity PA, and reduce sedentary behaviour in children and adolescents, however few studies have explored wearables in a family-setting (n=3 identified in our previous systematic review). The aim of this study was to explore the acceptability and feasibility of wearables to increase PA in 5- to 9-year olds, in a family-setting. Families were recruited via social media, staff emails and existing connections, and were eligible if they had: at least one child, aged 5- to 9-years, living in the same household, a smartphone/computer device with Bluetooth/USB port, and the internet/Wifi. Physical activity and perceived barriers of wearable use and PA were measured in week one and five. Family members wore the ActiGraph GT3X+ on their right hip for 7-consecutive days, and a parent/guardian completed a questionnaire (20 questions), which was developed using the COM-B behaviour change model, and Theoretical Domain Framework, and explored families’ capability, opportunity and motivation for PA, and using a wearable to increase child PA. Family members wore the Fitbit Alta HR for 4-weeks, and completed weekly surveys, which assessed the acceptability of using the Fitbit. Family members participated in a focus group to discuss their experiences using the Fitbit. Data collection is currently ongoing, with (to date) 12 families participating (n= 15 parents, n= 14 target children, n=5 siblings), and six families expressing interest in taking part. A thematic synthesis will be conducted for qualitative data, and changes in perceived acceptability (weekly surveys), barriers to wearable use and general PA (pre- and post-barriers questionnaire), and PA levels (pre- and post-ActiGraph data) will be presented. Statistical tests will not be conducted, given the primary aim of the study is to assess feasibility, and sample size calculations were not performed. By the time of this conference, at least 12 families will have completed data, and their findings available to present. The findings of this study will inform the development of future family-based wearable interventions. The use of the COM-B model and Theoretical Domains Framework to develop the barriers questionnaire, enables researchers to use the Behaviour Change Wheel to identify future intervention functions and policies that may increase PA.