More hindsight. 3/4" fasteners are too long for a 5/8" chine log with 5mm plywood. No room to set the fastener below the surface of the plywood. The nails or screw points will come through. Use shorter fasteners or at least a 3/4" log. Hillman makes a 5/8" exterior screw that would work well.
Wood flower can be mixed with Titebond Glue to use as a gap filler or hole putty.
A solution for the bottom issues has been decided. The boat has been passed to a new owner. He will be finishing the bottom with epoxy and 6 oz. fiberglass inside and out.
Using thicker plywood for the bottom would have been the best way for the boat to remain kid safe and buildable. The experiment was fun and worth doing.
I did not want to invest much more time or work into this project so I built this simple seat/bench.
The bench could serve several different uses for a outdoors person. A seat for starters , a fish cleaning station or a foot rest , a cooking table or a dinning area , a night stand for valubale inside a tent. The list goes on and on.
Chuck I like your ideas about the seat/bench.
I joined two boards together to make the bench wide enough using an idea from a video Joey mentioned to me. This is how I did it. It worked fine but a word of caution. Do not try to saw into the lumber you could ruin a blade.
Drove galvanized finishing nails into one board and cut the heads off.
Aligned the two boards with glue between them and hammered together.
After clamping and the glue dried they were very flush and strong.
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