Adults can calculate probabilities by running simulations and calculating proportions of each outcome. How does this ability develop? We developed a method that lets us bring computational modeling to bear on this question. A study of 40 adults and 31 4-year-olds indicates that unlike adults, many 4-year-olds use a single simulation to estimate probability distributions over simulated possibilities. We also implemented the 3-cups task, an established test of children’s sensitivity to possibilities, in a novel format. We replicate existing 3-cups results. Moreover, children who our model categorized as running a single simulation on our novel task show a signature of running a single simulation in the 3-cups task. This signature is not observed in children who were categorized as running multiple simulations. This validates our model and adds to the evidence that about half of 4-year-olds don’t evaluate multiple candidates for reality in parallel.