Making Apple Watch the default device for a Find My profile

My son wears his cellular Apple Watch SE to school, the paired iPhone 8 stays home with me. I want to track where he is with the Watch on, I don’t care where the phone is.

I can’t get Find My to notify me when he arrives/leaves school etc with the Watch on. It tracks both the phone and the Watch on the map, as him, but will only send me people notifications for the phone arriving/leaving (which is irrelevant for me).

His “Me” profile says “This device and cellular Apple Watches”. I want to change it to just the Watch, to see if that helps, but it appears to be an unchangeable field, on both the phone and the Watch. Help!