I have used 1 1/2 x 3/4 WRC for gunwales, and 3/4 x 3/4 WRC for all the other stringers. You might choose a heavier keel if you think the boys may not be careful.
If you have to splice together 8' boards for stringers and gunwales (like I do) I suggest a scarf joint (about 8/1 ratio) using epoxy (not 5 minute stuff).
Use 1/2" ply (marine) for the frames. I tried thinner and had the #3 frame buckle across the top.

I also use 8oz dacron from George Dyson, but there are other sources of similar material. Personally I think 8oz is heavy enough. Minwax Spar Varnish works well for a coating and is relatively cheap.

I personally glue the stringers to the frames with epoxy, but it seems like lots of people don't.
I also coat the edges of the frames with epoxy to seal the cut grain.

Good luck, these boats work well.
If you would like some build pictures let me know, but I do several things differently than others.