Unless you're running into some white water I don't think you'll have anything but fun.

When our kids were young we took the floating and put them in their own canoes and let them learn how to paddle by doing it. The life jackets you have will be fine as long as the fit well.

I wouldn't use a double paddle in a canoe. It's easier to push off the shore with a standard paddle and with a bit of practice you'll almost never have to change the side you're paddling on. The trick to going straight when paddling on one side of your boat is to twist the paddle a bit at the end of the stroke and push it away from the boat to move the boat back straight. I think the name for this is an "S" stroke, but I may disremember that.