Look into Hopkins Village for a few nights too. The beaches are great for swimming, and there are resorts with kayaks and beach bikes, along with restaurants. We stayed at a smaller place than might work for your family ( Beaches & Dreams), but we walked up the beach a bit and ate dinner at Belizean Dreams ( Belizean Dreams Resort) that might be a good place for a larger crew. There are other resorts in Hopkins that might work for your family (check tripadvisor for reviews) too. You can take several different tours from this area as it's pretty centrally located. There's zip-lining at Mayflower Bocawina National Park, river tubing at Cockscomb Basin Wildlife Sanctuary, hiking, and river tours on the Sittee River.

San Pedro is great for diving, fishing, dining, and beach biking. San Ignacio is nicely located for tours to Maya ruins. And let's not forget Placencia (or nearby Maya Beach), which also has beautiful swimming beaches.

Yes, you can definitely spend 1-2 weeks exploring Belize -- easily! You can spend a month or more if you have the time and money!

Former Belize expat traveling the USA & Mexico
http://travelingtwosome.weebly.com