To explore family and clinical factors for usage of an online serious game designed to prepare children with ECC for dental treatment under general anaesthesia.Observational study. Secondary data of 60 children, aged 5-to-7, randomised to the intervention group in a phase-III randomised controlled trial [NIHR Portfolio 10006, ISRCTN: 18265148] testing the efficacy of the serious game http://www.scottga.org (available online). Usage was captured automatically, with each click, in real time. The total number of replays and total number of missing slides per game-run performed by the child, were recorded and used to monitor usage. Compliance outcomes were: total time running the game and number of completely missed slides.57/60 played the game. Median age of parent/carer was 32. For 74% of the families, fathers resided at home and for 65% the parent/carer had A-levels-to-university education. At recruitment, 70% of the children were reported as anxious/highly-fearful and 37% as "significantly psychologically disturbed".Factors for non-compliance were absence of a father at home (P = 0.01) and higher child-anxiety (P = 0.01) and, to a lesser extent, a low parent/carer education level (P = 0.09). Interactive cartoons featuring dental assessment, oral health messages and modelling featured in the more popular slides.