I think Xanadu is excellent for families. Very nice condos (though a bit strange looking from the outside.) If you want something with larger units, to handle three teens, you might also look at Banyan Bay. The units there are two-bedroom, two bath plus a sofa in the living area, and they are large, over a 1000 square feet.
--Lan Sluder
San Pedro Cool, the Guidebook to Ambergris Caye