We have some great snorkeling trips that are very kid friendly. Some of them include swimming with sharks and rays, others include lounging off Goff's Caye, etc. If you do not want guided activities, you might want to rent come canoes. Our neighbors also have some dynamite mini-sailboats for rent that can be quite fun. Dorothy Beveridge can also do some educational slide shows for your little ones - some on reef ecology or rainforest ecology.
The Split is also an awesome area for your young un's to swim on a hot day. With a football (soccer ball to you), they can have loads of fun at the field, or even shoot some hoops at the basketball court. Bike rentals are $2 U.S. per hour, and golf cart rentals are $10 U.S. per hour. Sightsee a little bit if you are so inclined.