Whereas US-based data have contributed to our understanding of family composition changes over the last decades, data on Canadian families are limited, and previous studies have stressed the need for in depth, longitudinal investigations. This article begins to fill this gap in the literature by providing a current and detailed portrait of family composition changes from 1996 to 2008 (Study 1). Additionally, we performed an analysis of the role of specific child, parent and family characteristics, in interaction with family composition and family transition, in predicting pre-school children's behavioral outcomes (Study 2). Using nationally-representative Canadian data collected from the National Longitudinal Survey of Children and Youth (NLSCY), we focus our inquiry on a mean sample for 0--5-year-olds of 2,866 children at cycle 8 (2008). Results show increases in non-traditional family households over time, as well as significant relationships between child characteristics, household characteristics, and family processes in predicting three behavioral outcomes: emotional problems, hyperactivity/impulsivity, and physical aggression.