My kid found work arounds for that. For example she reset the ipad's clock to give herself more time. It really is like a drug to these children and they will lash out at you if you take it away.
The Fix: Lock it Down:
Go to Settings > Screen Time.
Tap Turn On Screen Time (if not already on) and set it up for your child.
Tap Use Screen Time Passcode and create a passcode that the child doesn't know.
Go to Content & Privacy Restrictions (under Screen Time) and turn it on.
Tap Allowed Apps (to disable camera/other apps if needed) and Change Restrictions.
Under Content & Privacy Restrictions, find Date & Time Changes and set it to Don't Allow Changes or Require Password.
Simplest example to point this out is if the parental controls only had things to limit phone usage generally without per app controls.
Hell, same goes for adults...