Hi! Do you mean that the app is asking for a parent’s permission to access it? If that’s the case, please go to: account.microsoft.com/family/home
There, sign in with the child account, click the three dots in the top-right corner of the child profile, select Manage consent, and then choose Remove consent. If you’re able to do this, afterwards try to leave the Microsoft family using the same three-dots menu by selecting Leave family group.