Recent work has documented that despite preschool-aged children's understanding of social norms surrounding sharing, they fail to share their resources equally in many contexts. Here we explored two hypotheses for this failure: an "insufficient motivation hypothesis" and an "insufficient cognitive resources hypothesis." With respect to the latter, we specifically explored whether children's numerical cognition--their understanding of the cardinal principle--might underpin their abilities to share equally. In Experiment 1, preschoolers' numerical cognition fully mediated age-related changes in children's fair sharing. We found little support for the insufficient motivation hypothesis--children stated that they had shared fairly, and failures in sharing fairly were a reflection of their number knowledge. Numerical cognition did not relate to children's knowledge of the norms of equality (Experiment 2). Results suggest that the knowledge--behavior gap in fairness may be partly explained by the differences in cognitive skills required for conceptual and behavioral equality.