Assessing young children’s physical activity and sedentary behavior can be challenging and costly. This study aimed to assess the validity of a brief survey about activity preferences as a proxy of physical activity and of a 7-day activity diary, both completed by the parents and using accelerometers as a reference measure. Thirty-four parents and their children (aged 3–5 years) who attended childcare centers in Sydney (Australia) were recruited for the study. Parents were asked to complete a 9-item brief survey about activity preferences of their child and a 7-day diary recording the child’s physical activity and sedentary behaviors. Both measures were compared with accelerometer data collected from the child over the same period as the diary survey. The findings suggest that parent completed diaries have acceptable correlation coefficients with accelerometer measures and could be considered in future research assessing physical activity and sedentary behavior of children aged 3–5 years.