Parents and non-parental adults who interact with children influence child development. This study evaluates the knowledge of child development in two large and diverse samples of adults from Alberta in 2007 and 2013. Telephone interviews were completed by two random samples (1,443 in 2007; 1,451 in 2013). Participants were asked when specific milestones in physical, cognitive, emotional and social domains typically occur (age birth to six years). Data were analysed using univariate and multivariate techniques. Linear logistic regression models were used to examine factors associated with knowledge and differences between the two samples while adjusting for differences and covariates. Adults' knowledge was overall poor (<25% correct responses), but notably higher for physical (48% in 2007, 44% in 2013) than social, cognitive or emotional milestones (=21% in all cases). Knowledge was lower in the 2013 than the 2007 sample (23% in 2007, 21% in 2013), and in particular for mothers after adjusting for location and income (24% in 2007, 22% in 2013). Evidence of low knowledge may have important education and policy implications. Factors such as availability of child development evidence-based information, parental stress and access to quality childcare might play a role in the low levels of knowledge.