Outside of Educational or Business environments, an iPad is essentially a single-user device. Sharing an iPad between users is fraught with issues concerning privacy and security.
In the circumstances, I suggest that you begin by completely removing the current ScreenTime Passcode:
Settings > ScreenTime > Change ScreenTime Passcode > Turn Off ScreenTime Passcode
Having turned-off the Passcode, you can now create a new ScreenTime Passcode known only to you. Write this down someplace safe - that your child won't be looking or otherwise discover (consider disguising the recorded Passcode, perhaps as an element of a phone number or address):
Settings > ScreenTime > Lock ScreenTime Settings - set the new Passcode
When prompted to set ScreenTime Passcode Recovery, tap Cancel - and then Skip.
You have now set a new ScreenTime Passcode, known only to you. There is now no way to bypass, reset or recover a forgotten ScreenTime Passcode on this device - and this is why you have safely recorded the Passcode elsewhere.
Should you need to change any ScreenTime settings, you will need this Passcode. If making changes, ensure changes are made out-of-sight of your child.
Never underestimate the resourcefulness and cunning of a child in discovering Passcodes. 🙂