Mike- I have to agree with the other guys... check out the Yost designs. They're outstanding boats, and not difficult to build. I did a 'dads and kids' build a few years ago, and the four Sea Pups we built are excellent kayaks, and it was a great project. I wrote up some instructions for building the boats with ply frames and wood stringers, and I'm adding a link to that site. There are also a couple of photo albums on my site with lots of building photos. Tom Yost has a construction manual online, with lots of great information, and I understand he'll be adding a lot more wood frame construction instructions soon. (Google 'Yost kayak', and you'll find it easily) The Sea Pups are great boats, but by the time our boys had reached 13 years old, two of them had outgrown them. (I'm 5'-7" and 140 lbs, and I BARELY fit- two of those dang 14 year olds are quite a bit bigger than me) I'm not sure which of Yost's designs to suggest. I really like the Sea Rider, and hope to build one for myself someday. Whatever design you choose, the costs and construction methods won't differ much. Canvas is kind of cool, and quite cheap, but check out the PVC that Tom recommends. That stuff is really tough.
Good luck, and enjoy the project!
Cheers- Steve Phillips
PS- if that pic doesn't come up, it's the first one in the album called 'Kid Kayaks'on my site.