Yes, by Caribbean standards Ambergris Caye accommodations, with only a few exceptions, are extremely reasonably priced.
Among the great places for families are:
Villas at Banyan Bay -- two bedroom condos with jacuzzis, great pool, nice beach. My family and I stayed here a few weeks ago and we all loved it.
Banana Beach -- if you can fit in a one-bedroom suite, this is a great place. It's one of the best values on the island, and the seafront units on the second and third floor are fantastic, right on the water. Friendly staff, okay pool.
Sunset Beach -- if you need a LOT of space, this is one of the few condotels on the island with three bedroom units. They're new (actually under construction right now). Nice management, pretty far south.
Gaz Cooper's Playa Blanca. New small condo set-up on North Ambergris. I was really impressed by what Gaz is doing here.
There are other good condotels, too, all over the island. The Palms ... Belizean Shores ... Mayan Princess ... and others.
One of the nice aspects for families is that the island is pretty safe, and you can drink the water and eat virtually anything without concern about getting sick.
You can make day trips to the mainland from San Pedro, but if you really want to see the mainland, which is 98% of Belize, I'd encourage you to stay there a few nights, preferably at a jungle lodge.
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