We stayed at Casa Caracol in January and found it to be a great location at a great price. It is within walking distance to the heart of town and right on the beach. It has 3 BR and 2 baths.
Marie and Phil were great hosts, they can set up any day trips you are interested in and they were quite helpful in answering any questions you might have about what to see and do.
I would highly recommend this place, sounds like it might be a good fit for your family.
Check it out on the lodging page of the website.
One trip to Belize and you will be hooked, just like the rest of us!