Greyed out childs icloud account on iphone

I can't guarantee a fix, but see:

Why is my Apple ID setting greyed out on iPad?  https://discussions.apple.com/thread/8355468.

A post in this thread says: "I just found the reasonfor my Apple ID Settings being greyed out. When I disabled Restrictions, my Settings became enabled and I was asked to update Apple ID Settings. S O L V E D...

Apple ID banner grayed out https://discussions.apple.com/thread/8611970. This one reports a solution also having to do with restrictions.

 My Apple ID is Greyed Out in Settings, Fix https://macreports.com/my-apple-id-is-greyed-out-in-settings-fix/

This occurs “most likely … because of a problem that happened when upgrading the iOS or when you change your Apple ID and password.”

So, you might use What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

Get a call from Apple support to help solve your problem using this

link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

To get Apple Support for help with your problem, use:

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter  https://twitter.com/AppleSupport

Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.