4 digit passcode

We are carers for a boy with special needs.

His previous carers set up an iphone and ipad for him but he (and they) can't remember his screen time 4 digit passcode.

He can use the phone and we obviously know his 6 digit passcode to switch it on and his apple id

Is there a way to reset the 4 digit passcode on an iphone 7 or do we need to factory reset the phone, and will the passcode be the same for his ipad - ie are they linked to the account or just the device?

Thanks