The only excursion available for little kids is the zoo and Howler Monkey Sanctuary. This tour involves about 3 hours on a bus but the tour guides were excellent. At the monkey sanctuary you only have about 15 minutes with the monkeys but it is amazing how close you are. The zoo is small by American standards but they have unique local specimins. Really my favorite part of this tour was driving through the "real Belize", outside of the port area. Belize does not have a welfare state, so even though it is a developing nation, small business owners and entrepeneurs are everywhere. The cashew wine sold at the monkey sanctuary is a unique local experience.
Online it looks like a wonderful beach park with water slides and playgrounds for children. But the sea is rough and deepens quickly. Everyone has to wear a life jacket to swim out to the water trampoline. You probably need to be at least 12 years old and a strong swimmer to enjoy the in-water amenities. Food and drink service was excellent, though the buffet was a bit "American-friendly" for my taste. Fruit, mashed potatoes, fries, etc. with just a few Mexican offerings.
A gem. Shallow and extremely calm beach, though you arem't allowed to go out very far. Perfect for small children; we were totally comfortable letting our 6 year old go out to the water while we watched from our beach chairs. The best snorkeling from shore I have seen outside of Hawaii, and several interesting birds and big cats.