Family sharing screen time passcode will not reset.

Here is the history. I created a screen time passcode for my <13 year old's ipad, prior to taking up the option of family sharing.

Sometime later I added <13 year old ipad to family sharing. I tried to reset the passcode for screen time and it will not reset. I can add the new screen time passcode but it doesn't overwrite the old one and I have forgotten the old passcode. I have tried to reset the entire device but it requires the screen time passcode. I have signed out of the <13 year old account signed in on my account and then resigned in on the <13 year old account. No of this makes a difference

I can't remove the child from the family sharing account because he's <13.

Is there a way to get round this screen passcode?