Device restiction code is forgotten

You would need to restore device with iTunes and set up as new to remove restriction passcode.

It's the same situation as losing device passcode except that you shouldn't restore from backup because that would bring the restrictions passcode back.

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

You can try the below. If it doesn't work, see the above post.

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

Hello,

I have recently had this problem. Do you have access to an Apple Computer with your Apple ID signed in?

Verification code - how to get one and sign in with two-factor authentication