The human capacity for rational decisions hinges on modal judgment: the discernment of what could, has to, or cannot happen. This ability was proposed to be a late outcome of human cognitive development, contingent on the mastery of linguistic structures. Here, we show that preschool-age children are capable of sophisticated forms of modal judgment. In two experiments, 96 children (aged 34-65 months) helped an agent attain a benefit or avoid harm. Consistent with logical distinctions, we found that children perform best when faced with choices that cross the logical categories of necessity, possibility, and impossibility, while they struggle with choices only differing in probability. Our results reveal that preschoolers spontaneously recruit logical concepts required for modal judgment, which likely predates modal language.