When adult children move back home, they’re guests, preferably paying guests.
I used that very same term...”guest”...with my kid when finished college, decided to take a year off from school and spend it at home. Didn’t go over with her very well. I stood my ground.
Our 25-year old daughter came home for Christmas and was in her room. I told her next time she came back it might be changed into a guest room.
“That’s fine - as long as I can still be a guest!”
“You’ll never be a guest here - you’re family, and always will be.”