Changing Child icloud accounts on device

I have two ipads that my two children use that are setup to one child account in our Family account (under 13 years old). I created a new icloud account for one of my kids and want to put that account on one of the ipads (sign out of the current/original icloud child account) and keep the other device with the original child account. When I go to settings and try to signout of the icloud account on the device, the box is grayed out and I can not figure out how to make the change. Any advice or guidance?

Thanks!