I must have not undertood.

Go with hard chine. You could build 3 frames of slight variation to fit 3 different body types. Smaller in all dimensions for for smaller or younger boys, a standard size and a wider boat for the bigger kids (and nice n stabile for fishing). The boats can then be built over the frames and the frames re-used. Just a thought.

Here's a picture of 2 of Mikes boats at Y2K. Sorry, his S&G Walrus isn't in the shot. Looks like an artic Tern to the left. Flip the frame only boat over and wrap with pieces of door skins. Stitch-it, glue-it, pull it off and start another. Finish the first boat with more or less semi-conventional S&G and Wah la! Boats