I'm embarking on a similar hands-on exercise with my own 5 year old son called "let's make an iPhone game". I'm letting him completely define the plot, the characters, the actions, and overall requirements no matter how nonsensical it all seems, I don't debate his choices, I just take requirements as is. I have him involved with every step from designing the artwork, the music, the sounds, the actions, and he is quite obsessed with working on this project continuaously. I don't expect he'll know all the details of how the game works but I want to demonstrate that he can create fun things using a computer.
The refreshing part of getting software requirements from a child is there's no tedious debate about implementation details, timelines, and business value. "The cowboy is only on level 2 and level 4 because his friend is the mosquitto." OK, whatever, nonsense but perhaps no more ridiculous than Pac Man.