Are you fishing a lake or a river? Are you allowed to use outboard motors where you fish, if so any HP restrictions?
I take my 6 year old lake fishing in my boat. It's a 16' Lund with a 25HP motor. It's deep enough that I don't have to worry about him falling out real easy, and it's simple to fish out of. Even with that, I still make him wear a life jacket at all times.