: Thinking of a kayak with sailing capability.

: John

You may wonder why we're slow at answering your question but the problem is we don't know about the needs that are behind your design choice.
We take for granted you want a kayak.
The question is why you want to combine two different propelling devices: the sail and the paddle.
Is the sail for the fun or is it for safety? You may want your son to be able to sail back home when he's too tired to keep up paddling e.g. in chop.
Good idea, but it all depends on the size of that inner lake and the strength of the young paddler.
The problem is that the shape of the hull and the kind of propulsion can't be disconnected when choosing a design.
Klepper built a multipurpose boat years ago that could be rowed, sailed, powered. It result was a terrible hull.
Sailboats are large compared to kayaks. No wonder: they have to sail upwind i-e tack and heave strongly one one side.
A downwind only kayak sail is easy to fit but what if the paddler is exhausted on a windy lake and has to sail back home upwind? He'd keep drifting and then have better get to the nearer shore of the lake.
Or you'll need a pair of leeboards, or at least one side leeboard, and a rudder. Not easy to steer one's boat with a paddle in a storm.
A pair of floaters, appending from a transverse yoke, would greatly improve primary stability, but the yak would be harder to right back if capsized.
Do you want a skin-on-frame, a strip-planked or a stitch and glued construction?
The last is generally considered the easier to build as a father & child project, but it doesn't look as good.
Building a sail rig is easy, especially if you just want a downwind sail, though I wouldn't recommend this on a large or stormy lake.
A pair of 2$ broomsticks will do, the connection ring or hook can be found at a home depot or the likes, notably in the house-building dept when there's one.
Aluminium brooms will give a high-tech look to your rig :-)
Dacron is a good choice for the canvas. Plan Cheap is car tarp or truck tarp.
Nylon is a more expensive possibility. Cotton soaks and stretches, should be fitted only to character boats.
Batsails have merit, as well as lugsails (Micmac Indians would use them, perhaps on a tip by Jacques Cartier, as well as Inuits on Umiaks).
Two gaffs going up sideways make a very efficient vee-shaped crab sail on downwind use (leaving an awful vortex behind the canvas).
I've tried to convert them into upwind sails but with little success so far!
Please see also the links below, and the other one you got from another response.
In the end, as for small boats: good paddlers are bad sailors, and vice-versa.
When the wind goes on strike, sailboats are better rowed than paddled, except for narrow pirogues which are unsteady and not tremendously safe.
Cheap, dark plastic oarlocks can be found at the shipchandler's.
Colossal sites here, in German but showing good sketches:

http://home.t-online.de/home/derpoly/mehrrumpfboote.html#oben

http://derpoly.bei.t-online.de/basteltips.html#oben

Sorry for the rant. Hope this helps.

Eric