AT&T offers one, as do a number of other vendors. Their tracking service which uses the kids phone - less secure than something attached to clothing, but good enough for most kids, is something like $9.95 per month.
Between autism and Alzheimer's, tracking modules would really help local authorities. I'm sure if the government is involved it will cost a lot more. Garmin makes a really nice device for dogs. If you get the government involved it will only cost 5 times as much. Two words for you Obama Phone.