Monitoring will end automatically if the age is set correctly. If it isn't, do this:
In your device:
1. Go to the "Apps of Concern" tab in the parent app
2. In the top right corner drop down, select the device you wish to delete.
3. Click "edit (device name)" from that drop down and delete.
4. In the top right corner drop down, select the old child profile
5. Click "edit (child name)" from that drop down and delete.
** If you didn't delete all the devices from that name, it will prompt you to do so before deleting the child profile.
Then in the child's device:
1. In the child's device, go to Settings->Screen Time->Content & Privacy Restrictions and turn that OFF.
2. Go to Settings>General>Device Management and delete anything there, even if it has our name on it.
3. Go to Settings > (Child's Name) you should see something called a "saferkid mac". If you see one there, delete it.
NOTE: The mobile device management profile should be removed automatically when you delete the device, so if you cannot find Device Management in the child's device - that's because it's already removed.