Using RFID / Geocache tags to trace child?

A friend's husband absconded with their child, and I've been trying to help her find him.  She thinks that if the father's parents were given or found a toy containing a trackable tag--like RFID or Geocache or something similar--that they'd secretly send it to the child, and that could be tracked. She is totally heartbroken and desperate.  I don't think that such a tag could be traced remotely across country, possibly even out of the country, unless you were in the immediate vicinity of the tag.  I guess that maybe even a short distance might help if the police were investigating a tip and wanted to make sure it was him, and they had some app that tracked the tag. 

This isn't a case where there's any issues about abuse--it just seems to be about jealousy and possessiveness, and my friend has already agreed not to prosecute the father if he'll just bring the child back and agree to shared custody.   Is there anyone who can explain what kind of tag would work in a situation like this?  I'm also a little worried that the father might scan the toy through his GPS and find the tag.

Is there any kind of tag that would work in this situation?  Sorry I am so ignorant about this stuff.

Let's say that the police find the child.  I notify them that I sent a toy with a particular RFID tag to a friend of the father's, and could they scan for it? That would show that the friend had helped the father, Would the police need a particular app on a smart phone or some other piece of equipment to scan for the RFID tag?

Would GPS work any better for the above situation?

Your help is much appreciated, thank you!