We're looking at getting my nephew one of those GPS tracker phone watches right now. He's only four years old (and my son won't need one this early) but it's so that we know he's safe when he's with his mom. The watch has four buttons that can call home, but it looks like a normal watch. It also alerts the parents as soon as it is taken off the wrist. I feel bad, but his mom has a tendency to not bring him back and deny the visitation that he is supposed to have with his father. If his father wins custody we'll be buying one of these four hundred dollar watches.
For my own son, probably around the age he's old enough to stay at home alone, we'll get him a firefly phone that only calls mommy and daddy.