Grey options in "Content & Privacy Restrictions" Menu

Hello,

My 3 girls use an iPad mini 2 that suits well for children. Those 3 iPad were all running on the last iOS update available : 12.5.5

I've activated the guarding options available in "Screen Time" menu to limit and control what they do on their iPad. Anyway, till few months, all the options that should be available in "Content & Privacy Restrictions" menu were grey and impossible to modify. It happen in the 3 iPad. As if stop the Screen Time control, it won't come back.

Any reason of that ?