How do you restore a screen time passcode if it's forgotten? (and there is not option for "forgot passcode"

If you’re not running current iOS or iPadOS (15.5), update.

Assuming current iOS (15.5), you’ll need to use the iPhone or iPad associated with the Apple ID that supervised this iPhone or iPad; one of the Family Organizer’s devices.

That’s this case, from the Related Article linked above:

How to reset the Screen Time passcode for your child's device

If you’re using Family Sharing to manage a child’s account, passcode reset options won’t be available on their device. Use the following steps on the family organizer's device to reset the Screen Time passcode for your child.