I found a way after faffing around for ever and apple being completely unhelpful.
I too needed to sign out of my daughter's apple ID on her old iPad as she now has a new iphone and my husband's having her old iPad (ha! usually the other way around, I know).
I tried all the above suggestions: removing restrictions (settings / screen time / content & privacy restrictions / disable (so the top line goes grey).....) or content & privacy restrictions / allow - account changes plus all other changes.
No nothing. She is on my Family Sharing settings so I went through all this on my phone and on her ipad.
Still nothing. Kept switching them both off and on to reboot.
Still nothing. pretty fed up by now.
In the end I just went and removed my daughter from my Family Sharing and having done all the rest - bingo - I was suddenly able to sign out of her apple ID on her iPad.
Then I tried to set her back up on my family sharing but as she has just turned 13 and already has her ID set up this was impossible - or maybe there's a way but apple is such a pain-in-the-a I really don't have more time to faff away with this.
Just hope this might help someone else though... good luck and make sure you have a few hours spare first... maybe a big glass of something on the side (or cup of tea) to help you through.