Maui:
Sugar Cane Train -- hands down best attraction (outside pool and beach) for young kids.
Kids were in absolute heaven. Even their Thomas the Train Souvenirs are cheaper than the mainland?! Even kind of fun for the adults. ^
Kona:
I would skip Volcano. It is just a lot of driving and just not the same payoff for kids. Old lava tubes and park are nice but young ones have limited attention span.
I would recommend Punaluu Bakery. I think it is delicious.
Also dinner for adults at Merriman's. It is downright amazing and so reasonably priced. By far best meal I had on Kona. The cost could have been double and I'd be just as happy.