On Maui, we enjoyed the Westin Villas. Good amenities for kids (water slide, pirate ship pool, snorkeling beach) and generally just a nice resort. Cheapest option for that is to rent from a timeshare owner, there are a number who make a business of it.
I rented from this person:
http://www.denisetravels.com/westinmaui.htm
And it went well. Because you are more than 12 months out (which is how far out the timeshare owners can book) it would be easy to get preferred dates.
Gor activities, my kids loved the aquarium on Maui and the jelly fish tour on the big island. My 4 year old keeps asking when he can go for a jellyfish handshake again. There is also a parrot sanctuary on the big island that books via Airbnb that we didn't get to but I thought it looked cool. There is a zoo on the big island that is admission by donation that was a good combo with the waterfalls on the north side.