I agree entirely with Bryan. I have built Green Valley's Kipawa, which is a 16.5' tandem which could carry three on day trips or longer trips with two paddlers. It has the added advantage of being small enough to handle well solo with a kneeling thwart (in case his kids don't take to paddling as he hopes). If he wants to go a bit bigger, I'd recommend Green Valley's Matawa. Both of these designs are a bit tender, but very efficient with good secondary stability. I have found strip building to be easier (though more time consuming) than S&G.

A final note: aside from fishing and portaging, I am a lifelong canoeist who has converted to kayaking. The efficiency and ease of handling makes paddling much more enjoyable in my opinion. Teenagers in particular like to go fast. Consider a triple kayak - Pygmy has one, and someone stretched Nick's double to a triple. I'd rather paddle a triple kayak solo than a tandem canoe solo. Or build two or three of Nick's S&Gs from his free offsets - I've built it and have been very pleased with its performance.