Passcode Forgotten for Screen Time

Passcodes are stored only on the device, so they can't be recovered in the way that an Apple ID password can be. I'm afraid your only option is to erase the device, then restore it from a backup made before the passcode was set. Please see:

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

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