The Maui Ocean Center is possibly one of my favorite aquariums anywhere. I love the focus on local marine life (instead of imported species), the hands on exhibits, and the size - it's just small enough for young kids to be able to take everything in, but big enough and interesting enough that you can easily spend an afternoon. Many of the exhibits are hands-on, and my kids enjoyed them so much that they visited everything at least twice!
The indoor-outdoor nature of the exhibits and beautiful natural scenery make this a great destination even if the weather is spotty during your visit.
There's a (kid friendly) sit down dining room and an outdoor snack stand, or you can visit one of the restaurants in the adjacent shopping center. The Maui Ocean Center isn't far from the airport, so if you arrive in the moring, this might be a great place for the kids to stretch their legs before you drive to your hotel.