This study examined the psychometric properties of a questionnaire developed with the guidance of the socialization model of child behaviour to understand modifiable correlates of toddlers' physical activity and sedentary behaviour. Findings are based on 118 parents (33.7 ± 4.9 years; 86% female) of toddlers (19.3 ± 2.7 months; 48% female) from Edmonton, Canada in The Parents' Role in Establishing healthy Physical activity and Sedentary behaviour habits study (PREPS). The PREPS questionnaire encompassed 21 variables across the constructs of the socialization model of child behaviour. Of the nine variables assessed for internal consistency reliability, eight had good (a = 0.70) reliability. Of the 15 continuous variables assessed for 1-week test-retest reliability, 10 had moderate (intra-class correlation = 0.50-0.74) and 5 had good (intra-class correlation = 0.75) reliability. Of the six categorical variables assessed for 1-week test-retest reliability, two had fair (? = 0.21-0.40), one had moderate (? = 0.41-0.60), one had substantial (? = 0.61-0.80), and two had almost perfect (? = 0.81-1.00) reliability. Of the 12 sedentary behaviour variables assessed for convergent validity, 7 were significantly correlated with children's screen time, of which three were small (r = 0.29), two were medium (r = 0.30-0.49), and two were large (r = 0.50) effect sizes.