High levels of childhood obesity have been observed globally over the last three decades. Schools are promising settings to implement interventions aiming to improve health-related behaviours in children. The aim of this study was two-fold; (1) explore the feasibility of Happy Homework (HH) from qualitative investigations with teachers and pupils and (2) examine preliminary effects of the HH pilot on children’s activity patterns, sleep and dietary behaviours. Four schools in Lanarkshire, UK, were randomised to either the HH intervention (n = 2) or usual curriculum control group (n = 2). The intervention ran for 8 weeks and consisted of interactive curriculum-focussed health and wellbeing homework tasks. Primary outcome measures were intervention fidelity, recruitment rates and compliance with trial procedures. Secondary outcomes were objectively measured physical activity (PA), sedentary behaviour (SB) and sleep via ActiGraph GT3X+ and activPAL4™ accelerometers, dietary intake, and screen-time (ST). Children (n = 158) were provided with informed consent, and subsequently, 68 of those children participated in this study. A recruitment rate of 43% and retention rate of 93% was observed. In relation to fidelity, the intervention group teachers (n = 3) provided children with HH each week for the full 8 weeks of the intervention. Compliance for ActiGraph GT3X+, BMI, ST and dietary intake questionnaire outcomes was high ranging from 91% to 100%, while 67% of the sample provided valid activPAL4™ data for both baseline and post-intervention. HH was found to improve measures of PA and SB in children in 8 weeks. Before progression to an effectiveness trial, additional procedures should be considered to improve recruitment rates, compliance with outcome measures, and implementation of the HH intervention long-term.