Sorry, but I have to disagree with the comments above. If you have 10 days to spend in Belize you're doing yourself a great disservice by not experiencing what the mainland has to offer. While the day trips from A/C are great they're no different than a shore excursion from a cruise ship. By the time you've done 2 or 3 excursions you could have easily afforded a few nights in a hotel.

It's whole different experience when you stay in a jungle lodge and exploring is a lot easier and a lot more fun especially when you have a small child in tow (most day trips from AC last at least 12 hours, plenty of that being travel time.)

I would suggest 3 or 4 nights on the mainland and then over to AC for the rest of your trip. There are plenty of family oriented lodges on the mainland that will not only be fun for your 5 year old but will be educational and may instill a life long love of all things tropical.

Lamanai Outpost Lodge, Chaa Creek, Banana Bank, DuPlooys, Hidden Valley, Windy Hill come to mind as good bets but there are plenty of others out there in all budget ranges.