We have 2 sets of Motorola "Talkabouts." We use them everywhere. We bought the second set for when we are traveling with friends following in cars...every car gets a walkie. Plus the new ones have rechargeable batts which the old ones did not..but they all do work together. My son is border line the age for a cell but I am not quite ready to commit. He's been using the walkie when he walks to the ball field for baseball practice (so I can keep tabs on him). The range limit is 5 miles on them so they work great.
We use them in the park because I separate from them with the baby since he cannot go on the bigger rides. This way I don't have to sit right outside the ride and they can find me when they get off.