Cognitive aspects of children's executive function (EF) were examined as moderators of the effectiveness of parental guidance on children's learning. Thirty-two 5-year-old children and their parents were observed during joint problem-solving. Forms of guidance geared towards cognitive assistance were coded as directive or elaborative, and children's responses were recorded. Children were then assessed on an independent version of the same task. A parent-rated composite of working memory and planning was used as a measure of EF. Directive guidance by parents was associated with more child errors during the joint activity, whereas elaborative guidance was associated with better performance. Parent-rated EF moderated the relation, such that the relation between elaborative guidance and better performance was only significant for children with low EF. During the independent task, EF again moderated the relation between parent guidance and children's performance, such that children with low EF did worse when parents had provided more directive guidance; for children with high EF, directive guidance was associated with better independent performance. These findings suggest that the extent to which children's performance relates to different forms of parents' guidance varies, and elaborative assistance may be more helpful for children with low EF.