My son is 4 since march and he likes to play my cab. Favorites are Ms.Pac Man, 1942 Suzuka 8 hours. He also loves to play my original Double Axle cab, and is AMAZINGLY good at it (as long as I do the gas and shifting

). Ms.Pac Man, he really knows how to control the Ms., but he is only eating the power pills after eachother and tries to get the ghosts, he hasn't discovered clearing the field yet.
I say go for it. Make it sturdy and if he doesn't like it now, he will in half a year (or a year).
I took my son to a Retrocomputer fair (
http://www.retrocomputerdag.nl) and instead of nagging me all the time, he had great fun and said it was "cool"

He also played a mini-cab there

and he made me promise I would build one for him
Sorry....gotta go, the little man wants to play Suzuka.....