Welcome Mariusz,
Thanks for reaching out to Microsoft Q&A Forum. I understand you cannot sign in to your child account because the parental account process returns the “Something went wrong. It’s not you, it’s us” message.
Please make sure you already tried the Q&A Assist steps. In addition, please try the following.
Could you confirm whether the parent account can sign in normally to other Microsoft services? Also, does the error appear immediately after signing in with the parent account, or only when approving the child’s consent?
In the meantime, you can try these steps:
1, Confirm that you are using the same adult Microsoft account that originally provided consent for the child account. Only the account that originally granted consent can remove or manage that consent. Managing parental consent
2, On the child’s Windows 11 device, open Settings, select Accounts, and check whether a Verify option appears for the child’s Microsoft account. If it does, select Verify and complete the prompts. Troubleshooting Microsoft Family Safety
3, If you do not see the child when managing consent, check whether consent was originally provided by another adult or with a different Microsoft account. Managing parental consent
Looking forward to your update.