Using data from a diverse sample of low-income families who participated in the Early Head Start Research Evaluation Project (n = 73), we explored the association between mothers' and fathers' playfulness with toddlers, toddler's affect during play, and children's language and emotion regulation at prekindergarten. There were two main findings. First, fathers' playfulness in toddlerhood was associated with children's vocabulary skills in prekindergarten whereas mothers' playfulness was related to children's emotion regulation. Cross-parental effects were found only for mothers. The association between mothers' playfulness and children's vocabulary and emotion regulation was strengthened when fathers engaged in more pretend play and when children were affectively positive during the play. These findings show that playfulness is an important source of variation in the vocabulary and emotion regulation of children growing up in low-income families. They also point to domain-specific ways that mothers and fathers promote children's regulatory and vocabulary skills, and highlight the importance of children's positive engagement in play. (author abstract)