I give my 7 year old (he was 6 last season) the responsibility of our pre-trip checklist. He enjoys checking each item on the boat and then checking it off on the list.
Once under sail, I let him sit across from me in the cockpit and give him the tiller. He also likes to help with the sheets and halyards (he thinks he’s helping anyhow)
When he begins to get bored (which doesn’t take long at this age) I let him play with his PSP in the cabin, but that doesn’t last long, as we try to find exciting things for him to see (other boats, birds, planes, etc…).
He looks forward to lunch, and when the wind dies (as it does so often around noon on the Finger Lakes) we anchor and enjoy a nice swim/float, with life jackets, and a loooooong stern line to hold onto.
The key is to keep them involved, without letting them get bored!