Some thoughts, some what organized.
1. Use 5mm exterior luan for sides. 9mm for the floor. 1/4" BC exterior pine if the luan was not available.
2. Butt splices with plywood backings for the sides. The floor may can be spliced over a rib. Either way no epoxy/fiberglass needed
3. 3M5200 glue or some other "below the water line" rated sealer and fasteners for the chines, just for liability peace of mind.
4. Probably would consider 14' long with 12" sides, because most kids will need to go with an adult at first and they will soon grow into it.
6. A little more work but I would cut an arch from the sides to reduce the rocker.
7. There are other designs and plans that could work for a kids build. Here is a link about kids building a boat at school.
http://www.alaska.net/~fritzf/Boats/KidClass/Boatshop/shopmain.htm
This link has a little more info about the boat.
http://www.alaska.net/~fritzf/Boats/Wacky_Lassie/Wacky_Lassie.htm