Regardless of their age, kids will want to feel like they are part of the canoeing vacation and parents need to be prepared to accommodate them.
One trick is to paddle the canoe backwards from the way it was designed. The child sits in the front, which is actually the canoe's stern seat. By arranging the canoe this way the weight distribution is more even, which makes the canoe easier to paddle. Also, the child is able to sit on a much narrower seat, which makes it easier for them to reach the water with their paddle.