Background: Nearly 60,000 people applied for asylum in the Netherland in 2015, confronting the governmental structures and services with great administrative, logistical and service provision challenges. Refugee children’s psychosocial needs and wellbeing are often overlooked, and post-migration support is of pivotal importance.Methods: An easy accessible movement–based psychosocial intervention, called TeamUp, was developed for children aged 6-17 living in refugee reception centres. A mixed-method process evaluation was conducted of (1) implementation process, assessing attendance (n = 2183 children, and n = 209 children); (2) implementation quality, using structured observations at two time points) to evaluate facilitator’s (2a) individual-level fidelity (n = 81 facilitators); (2b) team-level fidelity (n = 22 teams); (2c) facilitators’ competencies (n = 81); (2d) trainee perceived self-efficacy pre-post training (n = 73); and (3) perceptions on implementation and outcomes, employing a survey (n = 99), focus group discussions and key informant interviews with children (n = 94), facilitators (n = 24) and reception centre staff (n = 10).Results: Attendance lists showed a mean of 8.5 children per session, and children attending 31.3% of sessions. Structured observations demonstrated 49.2% and 58.2% individual-level fidelity, 72.5% and 73.0% team-level fidelity, and 82.9% and 88.4% adequacy in competencies, each at T1 and T2 respectively. Main challenges that were reported included managing children’s energy regulation (e.g. offering settling moments) and challenging behaviour. Training participation significantly improved perceived self-efficacy for trainees. The facilitator survey demonstrated on average, high satisfaction and self-efficacy, low experienced burden, and high perceived capacity-building support. Qualitatively, TeamUp was positively perceived by all stakeholders and was regarded as contributing to children’s psychosocial outcomes.Conclusion: (1) Attendance and group size were lower than expected. (2) The intervention’s facilitator fidelity ranged from moderate to adequate – exhibiting a need for specific fidelity strengthening and capacity-building – while facilitator competencies were high. Trainee’s perceived self-efficacy improved significantly following a two-day training. (3) Facilitators expressed high levels of satisfaction, self-efficacy and support, and low burden. The intervention was positively perceived by all stakeholders and to have a positive impact on children’s psychosocial learning and wellbeing.