Even if this was on little kids, I don't like it - it's a parent's responsibility to know where their little one is - they shouldnt be relying on technology to do that for them.
Teen agers- i'm sure most teens could figure out how to disable or trick it - there's probably be 1000 websites up with instructions...
if you can't trust your kid, you've got a bigger problem than an electronic gadget is going to solve.
Now, for years I wanted one of these things for my boss. he had a habit of disappearing when we needed him the most...