Very big brotherish. Also dangerous.
I know it would give some parents peace of mind, but it would also relax them into being less vigilant. People leave their children unattended enough as it is.
Besides, what's to say that tag couldn't be removed by an abductor?
Who would have access to the locater? There are some parents who should not have access to their children FOR A REASON.
According to NCMEC (National Center for Missing and Exploited Children), The largest number of missing children are, from most frequent to least frequent:
1. Runaways
2. Family abductions
3. Lost, injured or otherwise missing children
4. Nonfamily abductions (in these cases, the child is at greatest risk of injury or death).
Picture this: the movie Minority Report where the wall display Ads were tuned to an individual person. How far will they go? Slippery Slope argument, I know, but that doesn't mean they wouldn't do it. We've already got customized ads based on our internet use (if you don't clean out those damn cookies). What's next?