Sign out is not available due to restrictions
This took too long to find.
iPad we had my daughter on. Could not sign out her account on the iPad. Also could not turn off ScreenTime on that iPad... had to turn off her ScreenTime from my adult account on my device <my_device> -> ScreenTime -> Family -> Daughter -> Turn off ScreenTime. Could have used a clue on the other device to globally turn off ScreenTime! FWIW this is probably turn for any device that is managed by ScreenTime.
iPad mini (5th generation)