I agree that Oahu has a little of everything and it’s all quite accessible to tourists. Which other island to do, to me, depends a lot of the age of the kid(s). Maui and Kauai both had a lot more non-lifeguard areas than Oahu (or at least, that we experienced). I definitely wouldn’t take small kids along the toad to Hana, as they get bored “seeing” and will want to “do,” but much of the sites aren’t super kid-friendly (rocky trails, etc.). Kauai is insanely gorgeous and laid back, with two very nice kid-friendly snorkeling coves and a tubing activity they might be able to do. It’s also more of a contrast to Oahu than Maui. That said, I was planning a return to both Maui and Kauai, but I’ve cut to just Maui for logistics and cost. Still want to return to Kauai at some point.