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The major differences in Belize are that the cayes in the north (Ambergris Caye aka San Pedro, and Caye Caulker) are close to the reef, typically 1/2 mile. Caye Caulker is much quieter than San Pedro and may offer lower prices, but is too sleepy for many. San Pedro is the busiest single area in Belize, with lots of expats, dive businesses, hotels, new building, etc.
The other main center is Placencia in the south of the country. Like San Pedro a hive of activity and new building, but a long boat ride from the reef. Perhaps a great place for boating (all of the bareboat charter companies have gravitated there) but far from ideal for diving.
There are several small islands near the reef that also offer accommodation and facilities, to various standards and prices. Four I can think of near Dangriga (a bit north of Placencia) are Tobacco Caye, Southwater Caye (at the northern end of the eponymous marine reserve), Coco Plum Caye and Thatch Caye (the northern end of the Coco Plum range, renamed). I've been to all of these and IMO the nicest is Thatch Caye, followed by the neighbouring Coco Plum Caye. Southwater Caye is way down the scale of attractive accommodation (they don't even have an on-site dive center) and Tobacco Caye is (pardon my use of this expression) a tropical slum. It does have an on-site dive operation, which like everything else is housed in a tattered plywood shack, but it's not a place I would want to spend even a single night. Forget it for a family holiday - it's only suitable for undiscriminating backpackers, and they find it's not as cheap as it appears at first glance.
So for a family diving holiday I'd pick either San Pedro in the far north (my base), Caye Caulker 15 miles to the south, or one of the two adjoining islands 9 miles off Dangriga, Coco Plum or Thatch. I like Thatch Caye and have stayed (and dived) there several times. I've not stayed on Coco Plum but it gets good reviews. The buildings on Thatch are more immediately attractive and I'm told they have less of a sandfly problem than Coco Plum. Both places offer only full board - there's hardly anywhere you can pop out for a burger! Food at Thatch Caye is superb home-style, can't speak for what Coco Plum provides.
You could even try a two center holiday, a while at one of the two northern cayes followed by a period at one of the two off Dangriga. I knew some guests recently who spent time at Portofino resort on Ambergris Caye a few miles north of San Pedro town (a lovely small, peaceful, up-market and highly regarded resort), then switched to Thatch Caye (where I met them). They loved both places, and the transfer was pretty easy.
From all these places it's easy to take mainland trips of various sorts. From San Pedro it's possible to do these by air (maybe also Caye Caulker) but most people use boats. Down south it has to be by boat. Visiting Mayan ruins, exploring underground rivers and discovering ancient Mayan sacrificial chambers, floating down an underground river on a truck innertube, zip lining through (and above) the tropical jungle, mountain biking in the mountains, or just browsing through local food markets and smelling the oriental spices of offer - all are easy from any of these destinations.
To misquote a current TV ad - "It's your holiday, spend it where you want to".