Hello, Tim and SarahB
Welcome to the Microsoft community.
Thank you for your feedback. It does seem likely that your child's login credentials are corrupted. You can send me a screenshot of the problem you are experiencing and this will help me pinpoint your issue.
You can try the following:
On your account, log into your administrator account, search for Control Panel in the search bar, click on User Accounts, Delete Account. To delete your child#3's account, it is recommended that you select Delete Files. After that, still on the User Accounts page, click Credentials Management and delete all credentials for your child #3. Restart your computer and make sure that your child#3's account is no longer on your computer.
After that you can add one of your child's accounts to your Settings, Accounts and see if the problem persists. Or try adding a local account and select I'm adding a user without a Microsoft account . After that click on Sign in with Microsoft account instead.
Manage user accounts in Windows - Microsoft Support
If the above doesn't work, it may be due to some system components and you can try to upgrade in place. This will refresh your windows and will not delete your data, but it is still recommended to back up your important data.
How to run In-place upgrade in Windows 11 - Microsoft Community
I look forward to your feedback.
Best Regards,
Aaron| Microsoft Community Support Specialist