Welcome Weslley Nascimento,
I understand how frustrating it can be when your son's Microsoft account appears to be stuck in a parental consent/family safety state, showing the message "This child already has parental consent for their Microsoft account," while the account still cannot be successfully added to a Microsoft Family group.
Before proceeding, could you please clarify:
- Does the child account appear in the Family Safety portal at https://family.microsoft.com after the invitation is accepted, even temporarily?
- Is the child account using the correct birth date and country/region in the Microsoft account profile?
I look forward to your reply so we can continue working together to address the issue.