I agree with Marty. Banyan Bay is one of the best condotels on the island, and very family friendly. Very large (1,000 sq. ft.) two-bedroom, two-bath apartments with full kitchen, living area and balcony/porch with sea views. Beautifully maintained, with A/C and lots of tropical hardwoods. I rated it four plus stars in my guidebook to Ambergris Caye (San Pedro Cool). Lots of kids there, especially in the summer and during holiday periods. It's on one of the best beaches on the island ... but your kids will probably spend most of the time in the big pool.
--Lan Sluder