Given increasing trends of obesity being noted from early in life and that active lifestyles track across time, it is important that children at a very young age be active to combat a foundation of unhealthy behaviours forming. This study investigated, within a theory of planned behaviour (TPB) framework, factors which influence mothers' decisions about their child's (1) adequate physical activity (PA) and (2) limited screen time behaviours. Mothers (N = 162) completed a main questionnaire, via on-line or paper-based administration, which comprised standard TPB items in addition to measures of planning and background demographic variables. One week later, consenting mothers completed a follow-up telephone questionnaire which assessed the decisions they had made regarding their child's PA and screen time behaviours during the previous week. Hierarchical multiple regression analyses revealed support for the predictive model, explaining an overall 73 and 78 % of the variance in mothers' intention and 38 and 53 % of the variance in mothers' decisions to ensure their child engages in adequate PA and limited screen time, respectively. Attitude and subjective norms predicted intention in both target behaviours, as did intentions with behaviour. Contrary to predictions, perceived behavioural control (PBC) in PA behaviour and planning in screen time behaviour were not significant predictors of intention, neither was PBC a predictor of either behaviour. The findings illustrate the various roles that psycho-social factors play in mothers' decisions to ensure their child engages in active lifestyle behaviours which can help to inform future intervention programs aimed at combating very young children's inactivity.