What if the locations I want to set are too close to each other(< 300 m), how would I know when my child leaves or enters these locations?

You could setup the geofences such that the second location doesn't fall within the first location's geofence. Adjust the 300 m radius so that both locations have a different boundary of geofence.

This way, you can receive notifications while they enter/leave both these locations

To learn how to setup a Geofence, check out How can I check if my child is really going where they say they are?