Hi, thanks for reaching out, happy to help!
After a recent Windows 10 update, this issue usually happens when the parent account does not load correctly after adding a child account. First question. Do you see your parent Microsoft account on the sign in screen or only the child account.
The parent account must sign in first so Windows can load sign in options like the password screen. If the parent account does not appear, I suggest using Windows recovery to reach Startup Settings and sign in with the parent Microsoft account, then check Settings, Accounts, Sign in options once logged in.