After changing screen time code, the old one is still working

After my son found out the screen time code for his iPhone & iPad, I changed it on my iPhone, but unfortunately both the old and the new code now work on his devices to extend the screen time. However, you can only access the screen time settings with the new one. How can I solve this problem? I have already reset the screen time on the devices several times and also restarted the devices, as well as set up the screen time directly on the affected devices and changed the code there several times, but when I then extend the screen time of an app, you can do it with both the old and the new code. What else can I do? Can I delete the code from the iCloud?