Our 11 and 14 year-old daughters can't wait to get back in March. If Dad's SCUBA certified, why don't you and the girls join him? We dive as a family (except for our 8-year-old who, actually, is quite good at it but not old enough to certify). There are some great and very safe sites for the family there. Amigos Del Mar took us out as just our family several times and I'm certain other shops would, too.
They'll have a great time and your resort can set up your snorkel or other trips for you. Parasailing with Tony at the pier at Fidos should be a blast for them, too. Driving the golf carts alone should make them happy!
The Lamani day trip to the mainland is a great family outing (my kids said, they went with their aunt while we went on the Blue Hole dive trip) and cave tubing was great fun. Walking the beach and wandering around town are simple and wonderful methods of entertainment. You can discover some great, inexpensive food at tiny restaurants doing that.
The chicken drop (just ask around). A full day fishing, snorkeling and bbq trip out on the reef is one of our favorite days as well.
They really won't run out of things to do. And there's always just hanging around the pool.
Mostly you need to know to relax. Downshift at the airport in Belize City and start enjoying the calm.
There's a jungle. On the mainland. A huge one. Fly over or boat over for at least one trip. Don't hang out in Belize City.
Somebody get me a Belikin, would you?