If you can afford it, I agree with getting your kids passports. I would then recommend the Guanacaste area of Costa Rica. Fly in to Liberia and then: stay in mountains for a couple of days - hike, go to waterfalls, tube the river, zip line, etc. They will have a blast. Then go to the beaches on the Pacific Coast: surf, surf, surf, visit different beach towns, eat some good food, take a charter out to fish, relax, etc.
I have three kids and we've been to CR about six times, going on seventh at the end of this month. Our favorite place to go/be. We gave my oldest son, who is graduating from med school, the choice of anywhere he wanted to go in Europe/North America/ Central America. Surprisingly, he chose to go back to CR. It gets in your blood. Pura Vida