This study examined the cognitive predictors and constraints of children's strategic offers in the Ultimatum Game. The Ultimatum Game is a one-shot negotiation on the division of a given sum of rewards between two people, where the receiver has the right to punish the proposer by rejecting unfair offers, which results in nil reward for either party. Ninety-four Hong Kong children ranged between three and six years of age (48 girls, M = 4.94, SD = 0.90) participated in an Ultimatum Game, and tasks measuring theory of mind, cognitive flexibility, inhibitory control and verbal ability. Children's theory of mind, measured with false belief understanding tasks, was positively associated with their Ultimatum Game offers. Regression analysis further demonstrated that cognitive flexibility moderated the association between false belief understanding and children's offers in the Ultimatum Game. Being able to take the opponent's perspective prompted children to offer more in the Ultimatum Game, but only when their cognitive flexibility was low. [Spanish translation by Mercè Rius.]