Always wonderful when a parent wants to build a kid sized kayak.

Couple of possible considerations:
--most people seemed confused about children's ages and the calendar.
A child is 9, 10, 11 and 12 (cute) for about three weeks each. They are then 13, 14 and 15 for about 8 years. (Don't worry, by the time they are 21, it'll all average out to 12 months per year--depending of course upon factors such as pregnancy(ies), arrest(s) and bail/fines/lawyers.)

Building a kayak to size for a 9-12 year old will be outgrown before you log back on and ask about methods to restore scuffs and nicks. Building a kid sized kayak will initiate an inch a month spurt more effectively than human growth hormone.

What everybody (OK, guys) wants the third day they go out in their new kayak is a longer faster kayak (pray you don't see any outriggers, we've lost a few kayakers to these 'cheat' boats. (If you're faster than me in my kayak and you are not half my age -- you must be cheating.) A 15' kayak is great until you want to paddle more than about 4 miles, or 2 miles with friends who are in 17' kayaks.

My parents got me a 24 inch wheel bicycle I could barely manage on. I lowered the seat, after a couple of years I had to buy a longer seat post. I wore it out. 20 inch bicycles like 12' kayaks sure are cute and last forever because no one rides them more than a dozen times. (Thank you in advance everyone who replies to tell me how often their kids enjoyed their 12 footer. I stand corrected. How about those of you with 12-15' kayaks that aren't being used loan or rent them to the others wanting to make short kid'yaks?)

The only non-snarky suggestion is to make sure your child works on the kayak. What we build is ours. Buy-in, take ownership, etc... is all true.

Better still might be a set of simple instructions on how to make a basic kayak out of scrap using almost no tools that kids could 'discover' and do all by themselves or with a friend.