We just got back from a 2 week trip to Belize and I thought I'd post here in case anyone is every wanting to research Belize. We had the best time. DH said it was the best family vacation we've ever done. Here is what we did and what we recommend.
Flew into Belize City and immediately were picked up by a driver from Black Rock Lodge
http://www.blackrocklodge.com/
We loved Black Rock Lodge and it was amazing. Howler Monkeys woke us up several mornings. It is rustic but really lovely. We went to a Mayan ruin and did a canoe trip down the river. We also went tubing down the river and had a blast. The food was excellent and the guides were wonderful. We had Black Rock drive us to Placencia in the south. In retrospect we COULD have rented a car but it is sort of a toss up. We spent 4 nights at Black Rock and for 2 weeks we could have done one more night here.
We then spent 10 days in Placencia at Chabil Mar. It was lovely. We rented a golf cart of the week and we went into town every day for meals and wandering around. We had a great time and it was more resort relax but in a developing country. We felt safe the entire time. Food is not cheap but we did some meals at our room (which had a full kitchen). We did ziplining on the way to Placencia and it was super fun. In Placencia we did a full day of snorkeling (with nurse sharks!) and went horseback riding for a half day.
http://www.chabilmarvillas.com/
All in all it was a lot fun, we did things we have never done before and we spent a ton of great family time together. The boys were/are 10 and 12 when we went.