Screen time passcode

From the article below.

To change or turn off the passcode on your child's device, tap Settings > Screen Time > [your child's name]. Then tap Change Screen Time Passcode or Turn Off Screen Time Passcode, and authenticate the change with Face ID, Touch ID, or your device passcode.

Parental controls

If that doesn't work, you will need to restore the device to factory settings. Do not restore from a backup made when the Screen Time code was active.

Restore to Factory setting

As passcodes are held only on the device, and for security reasons you need the passcode to reset the passcode, the only way forward is to erase the device. Please see

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support