When determining what others know, we intuitively consider not only whether they succeed but also their probability of success in the absence of knowledge (e.g., random guessing). Across three experiments (n = 240 North American 4-6-year-olds, data collected between 2020-2023) we find that 4-year-olds understand that tasks with a lower probability of chance success are harder. However, it is not until age 6 that children use this understanding to gauge (Experiment 1) and infer (Experiments 2-3) what others know. These results suggest that, although basic probabilistic reasoning and representations of knowledge are well in place by age 4, children do not integrate the two to make mental-state inferences until much later, pointing to an area of important developmental change in Theory of Mind.