Why not a nesting hard dinghy?
In their spare time, I'm sure the kids are going to want to take it out and swim off the thing - and drive it. I'd much rather have inflatable tubes around the boat so when (not if) they run into something I'm not forking out money for a paintjob repair. Plus, when you're on mooring I'd be much less worried about damage with the dink in the water tied to the stern. I don't want to have to put it back up every time we aren't using it.