"Io" Childs Kayak
Io is one of the moons of Jupiter. It is also a simple, fun kayak suitable for children up to about 75 pounds [36 kilograms] that can be made from a single sheet of 4mm marine plywood over the course of a weekend or two.
I originally made this boat for my nephew Peter when he was 5 years old. He took to it immediately. Since it is not meant as a stable boat the first thing I had him do was capsize. He enjoyed this so much that he did it another dozen times before heading out on a paddle. Being 5 this expedition amounted to out around the raft and back, but it didn't take him long to figure out how the paddle worked and how to steer and go straight. I made a very simple Greenland Style paddle for him out of a short piece of 2 by 3.
The construction of this boat is very simple. 3 pieces for the hull, 3 for the deck and a easy lamination for the coaming. The inside and outside are both covered in fiberglass and epoxy although you could probably get away without. 4 ounce glass and less than a gallon of epoxy is good. Built like this it is rugged and should last several generations. It is light enough that if you are in a kayak or other boat beside your child you can pick up and empty Io and help the child climb back in after capsizing.
My nephew did very well at 5 years old and got a couple years out of the boat. It should have enough buoyancy for kids up to about 80 pounds [36 kg].
As Io is meant for children, it is up to an adult to make sure it is used safely. Think of it as a large, somewhat heavy (for a kid), and really quite hard, pool toy. You should expect that they will capsize in this boat and you should encourage them to try capsizing right away so they know what it is like. It is probably a good idea to have them wear a life jacket at all times and be comfortable swimming. You will probabably want an adult very close by at all times.
The plans for this boat are available for free. The instructions are also free at Instructables.com for free or the printed plans come with a manual. You can also find a writeup of this build at Makezine.com