Family Sharing with daughter's new phone

I bought my daughter a new phone, but I am unable to add her to family sharing media and purchases. She has an app subscription that I purchased for her last year, but she is unable to get the purchased subscription on her new phone. I had her sign out of her apple ID on her phone. I signed in on her phone with my apple id and I was then able to see her device listed under my apple ID on my phone. She wants to be logged in under her apple id, however, and if I log out I am not able to see her phone listed under my devices. How am I able to keep her phone activated under my id, if she's logged in with hers? I was able to do it on my old phone that I gave to my son a while back and he is logged in under his apple id. I can't seem to do it with the iPhone 12 though. Help!!!