Arduino

It is hard. You need face detection to track children. You can try openC V and python.

Assuming that this is for legal reasons and not for any other reason, this project would be fairly simple to make. But then again is it morally to track what i assume to be your children? Not sure, that’s not my decision lol. Anyway, the device would have to have some way to connect to the internet and send a signal to a server when moved out of a specifed location