Prospective memory (PM) involves remembering to carry out intended actions in the future (e.g., posting a letter on the way to school or passing on a message) and is important for children’s independent functioning in daily life. This study examined, for the first time, the effects of incidental reminder cues on children’s PM. Five- and 7-year-old children (n = 160, 50% female, predominantly White from lower middle to middle-class families) had to remember to put cards with a picture of a dog into a box (placed behind the child) every time they finished working on an activity book with a line drawing on each page (activity-based PM task). Additionally, the picture presented on the last page of each activity book was manipulated to examine the role of incidental reminders on PM. Results showed that 7-year-old children significantly outperformed 5-year-olds on the PM task despite age-equivalence of performance on the ongoing visual search task. For both age groups, an incidental reminder (a line drawing of a dog) that was similar to the target of the PM task (a card with a colour picture of a dog) significantly improved PM compared to the no reminder condition (a line drawing of a flower), while reminders related to the PM action (a line drawing of a box) or semantically related to the target of the PM task (a line drawing of a cat) were not effective. These findings have important practical and theoretical implications and open up interesting avenues for future research.