I think Banyan Bay is a great choice for families. My family and I stayed there last summer and everyone loved it, and there were quite a few families there at the time, so my kids found friends to chill with. It's a little far to walk into town, or at least to do so regularly, but you can rent a golf cart or take a taxi (US$5). Also, there are some restaurants within walking distance, including Rico's at the hotel.
In Cayo, while Chaa Creek is excellent you might also look at Ek'Tun, which has the coolest swimming pool in Belize (a natural mineral water pool cut out of limestone, with incredible blue water -- usually it's just you and the howler monkeys).
I'd definitely try to make at least a day trip to Tikal. The Maya sites in Belize are all very interesting, but Tikal is in a class by itself.
--Lan Sluder www.belizefirst.com