You have to hand it to the kids. They are much more technologically advanced at their age than ever before. So advanced in fact, that there are already two workarounds for the time limits that their parents can place on app usage using iOS 12's Screen Time feature. Screen Time is designed to give parents control over their children's iPhone usage, allowing them to set a cap on how long Junior can use certain apps.
One way to break the cap is to turn the time and date back on an iPhone, giving the user extra time on an app. This can be done even if the phone is already in "down time." The other way is a bit more complex, but a 7 year old happened to figure it out. When this kid runs out of screen time and his apps are locked, he goes to the App Store and downloads a previously installed (but later removed) game using the cloud icon. And the game works without any issues!
Parents can put the kibosh on the latter "hack" by going into restrictions on Screen Time and turn off Install Apps. This will remove the App Store icon from the device. It also will prevent any updates from coming through since that does count as an installation, so keep that in mind.
Now that these workarounds have been made public, we could see Apple come up with a fix for both Screen Time cheats.