Hi,
We were in Belize this past November with our two kids (aged 10 & 7). Just wanted to add a few things to the excellent advice you have already been given.

- You may want to consider staying near the Belmopan area. We stayed at Pook's Hill, and loved it. It is really beautiful & there is a lot to explore. Also, it is closer to Actun Tunichal Muknal (I know - I butchered the spelling!) and Caves Branch than San Ignacio, so you would have less travel time.
- We took a day trip to Tikal. It was one of the highlights of our trip - BUT - it is an all-day venture. If you have only two days, you may want to consider some closer ruins. (By the way, Pook's Hill has a small ruin located just behind the cabanas.)
- I have heard that the political situation in Guatemala is getting unstable right now, and there have been isolated incidents of tourists being stopped on the road to Tikal. I almost hate to say that, since Tikal is such a fantastic place, but - as a parent - I would want to be informed before deciding to take my kids.

Hope you all have a great time in Belize. It was a great vacation for us!