For the tubing you get picked up at the wharf by a couple of mini vans. When we did it there were about 20 people and they had 3 vans waiting. There will be no problems bringing your son along, it's a lot of fun and doesn't require any sort of skill or atlethic ability.
As for making your own way to the falls, I was glad I went but just as glad I didn't make it my major trip. It is kind of neat to see hundreds of people making their way up the falls.
My wife and I just booked another cruise for March and we've already booked the tubing.