We stayed next door at the Marriott, but my daughter did the Dolphin Quest when she was 6 and had a great time. The property looked very nice.

A day trip around to Volcano National Park, including a helicopter ride and a trip down to the end of the road to see lava flowing into the ocean at sunset were also well liked.

Oahu felt more big-city than vacation/resort - traffic, industry, etc. Pearl Harbor was a must-see for me, but I'm not sure it does much for the little ones.