If I set up a Geofence will it notify the child's device that its being fenced?

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?