I was considering a 3/4 x cross section,
: which gives an overall beam of 18" and a WL beam of
: 17.1". The OAL x 2/3 would be 11ft 4" or OAL x 3/5
: would be 10ft 2".

I would definitely go with the shorter length. 10' is plenty. I am an old 225 pound person and I frequently paddled a 10' 2" boat. There are many advantages to short boats. For a kid, there are very few advantages of a long boat.

I would less strongly recommend a beam of 19". It will make the boat more useable for more people for a longer time.

I would recommend a fairly stiff tracking design. At that short length it will still turn fine and it is frustrating not to go straight.

Hamm mentioned Leidy's software. I would also take a look at BearboatSP which works with Macs or Windows

http://web.mac.com/rlivingston/BearboatSP/BearboatSP.html