Yes. As long as you (the parent) have the Microsoft account credentials for the child's account (or your own account that manages the family group), you can activate it.

The screen will display a unique and instruct you to visit the activation link. Step 2: Use the Link to Authorize Your Console

The codes generated by your Xbox console are only valid for a short window (usually 15 minutes) for security reasons.

You may have forgotten your password, or two-factor authentication (2FA) is blocking you. The Fix: