The Family Monitor app works with the Google Maps shared locations function. Family Monitor allows for alerts to be set that are triggered when a family member becomes closer to or further from a location or another member of the family. For example, an alert can be created that is triggered if your child moves further than 100 metres from your location. Family Monitor only needs to be installed on the phone of the person doing the monitoring. Nothing needs to be installed on the phones of those being monitored. However, those being monitored must enable their location to be shared through Google Maps. (Those being monitored must proactively accept the monitoring and are regularly reminded by Google that their location is being shared.)
This is the full version of the app - an unlimited number of alerts can be defined.