Number 1, ask your child if they are doing anything nefarious to alter the settings. Trust is important with this one.
If that is not possible, there are instances where kids have avoided the screen time limit by:
- Deleting app and reinstalling. When an app is deleted, it will also delete the limits set on it. After it has been deleted, they could reinstall it and not have any limits on it until you set it up again. To avoid this, go to Screen Time > Contents > Privacy Restrictions > iTunes & App Store Purchases, and change the Installing and Deleting app settings to Don't Allow.
- They know your Passcode. It is possible they have watched you enter your passcode or even recorded the screen on their device while they had you enter the passcode for some made up reason. You know the screen is being recorded, when the time on top has a red box around it. You can stop allowing screen recording by going to Screen Time > Content & Privacy Restrictions > Content Restrictions > Screen Time Recording.
If those don't work, you may need to change their Apple ID Password and not give it to them and then go to Screen Time > Content & Privacy Restrictions > Account Changes and change that to Don't Allow. You can remove the apps that they are abusing and they will still be able to use the phone for emergency situations.