Do you know which Island you'll be going to, or looking at? There's Fitzroy, but not much there tbh. A bit of snorkelling, a cafe, that's about it. Green Island is a lot more family friendly. Google Big Cat Green Island Reef Cruises or Great Adventures, you'll get an idea what's there. They're the 2 popular ones. A friend of mine works on Great Adventures, has done for 9 years now. There is a Resort on Green Island, but to use their facilities (pool etc) you may have to be staying there. Most of the tour companies are owned by the same companies, so you're getting roughly the same deal across board.

ETA:
These 2 I mentioned are just off the Cairns coast. If you want to travel down a little further and stay somewhere for a couple of days that is kid friendly, then you can't go past Long Island. We loved it there. Very family friendly island with plenty to do for parents and kids.