The ability to reason about the difficulty of novel tasks is critical for many real-world decisions. To decide whether to tackle a task or how to divide labor across people, we must estimate the difficult of the goal in the absence of prior experience. Here we examine adults' and preschoolers' inferences about the difficulty of simple block-building tasks. Exp.1 first established that building time is a useful proxy for difficulty. Exp.2 asked participants to view the initial and final states of various block-building tasks and judge their relative difficulty. While adults were near-ceiling on all trials, children showed varying levels of performance depending on the nature of the dimensions that varied across structures. Exp. 3 replicated the pattern. These results suggest that children can reverse-engineer the process of goal-directed actions to infer the relative difficulty of novel tasks, although their ability to incorporate more nuanced factors may continue to develop.