This study examined the activation of the anterior prefrontal cortex (aPFC) in young children during block-building planning and explored whether activation levels and difficulty predictions varied by age. A total of 124 children, aged 3 to 6 years, participated in oneon-one interviews with a researcher. During the sessions, children’s aPFC activation was measured using the fNIRS method. Following the planning tasks, they were asked to predict the difficulty of the block-building tasks. The results showed that 5-6-year-olds exhibited significantly higher activation in the left aPFC during planning for an easy block-building task compared to 3-4-year-olds. Additionally, after planning, the older children predicted the easy task to be less difficult than the younger children and were more accurate in distinguishing between the difficulty levels of easy and hard tasks. These findings suggest that older children, who exhibit greater left-aPFC activation during planning, are better at predicting the difficulty of the tasks they plan than younger children.