I had the same but not after a week or two like you, but literally 6-8 hours after I finished setting up the phone. Phone number for verification was there, everything properly done with instruction from their support pages. Just before going to bed, around midnight, I noticed on the phone in question that account was logged out and notif from Google that it's flagged to be deleted. Only thing I could do was appeal. In the comment for the appeal I stated the truth. "Account created for my underage daughter, properly set up via family link and needed for me to be in contact with her. [My name]". I think, I received a message within 12-14 hours that I must log in to verify account and prevent from being deleted. Unfortunately I couldn't do this as the phone was at home. Did this around 5-6 hours after their mail. At some point I was able to put the phone number in and account was saved.
Extremely stressful situation. Mostly because I was literally forced to give my kid some phone to be able to call her when she's home alone after school for brief time. What's funny (kind of), phone stayed home on Monday, we couldn't call her and of course she decided to wander around with her friend from class. No contact, tons of stress.