Also make sure you bring something to occupy the childs time if the fishing is slow. My GF's 5 year old has an attention span of about 10 minutes without a bite. Once ten minutes is up, he is done for a while and wants to play.
Just like the others have said, if the bite is on don't plan on doing any fishing for yourself. Whenever I take the boy out I bring my rig but understand it may never even get used. Although when the kid is catching fish after fish, I don't really miss fishing myself. The smile and look on his face is way more satisfying than catching fish myself.