Andy, I was going to suggest a landing craft, as thats what I decided on for the lad when he was 9 or so. Easy, all flat bits, added advantage if prop made to fit slightly above the bottom line..can be used as a play thing in the house with toy cars etc. (added interest).. also of course allows toy cars to SAIL as well! neednt be military, could be a simple car ferry.
However Alun has generously offered some hulls which no doubt would be just the job as a good starter for easy boatbuilding.
Ashley