I found that giving my 4 year old 'jobs' that were his responsibility when we were preparing the boat, untying, underway etc. has made a huge difference. One of his jobs is to take off the tiller and winch covers and stow it in the lazarette. Utterly minor, but he takes it very seriously. He is also responsible for making sure everyone is wearing a lifejacket while on deck - and woe betide the skipper when he forgets. He also has important sail folding duties at key points.
He doesn't really have the strength for hauling on the sheets or any of that yet, and he doesn't really get steering (I tried), but these things will come with time. So far he is really, really into it - he loves when I take him down to the boat with me just to check on it or tinker. But then, he's pretty much really into anything - he's a 5 year old.