Kids will not know if Parents have setup a Geofence for their child's Android device. That said, there are a few ways that kids can "go stealth":
Turning off their phone.
Turning on Airplane mode.
On non-Samsung devices, disabling the location permission. (On Samsung devices, we protect the permissions from being tampered with)
On non-Samsung devices, modifying the location settings. (On Samsung devices, we prevent the modification of the location settings)
Location Reporting is automatic based on the "Check-in Frequency" so if the device has an old location time stamp, then you will know that something is amiss and can check into the above.
Related - check out: How do I setup a Geofence?