If you suspect the kids did install something, try booting your S5 up into its Safe Mode:
https://support.t-mobile.com/docs/DOC-28456
When running in Safe Mode, it's just a 'clean' Android without third-party apps getting pre-loaded. If you don't see that same graphical oddity in Safe Mode that would indicate some app is causing the problem. Which goes back to that problem figuring out just which app it is. If you open up the Play Store app, tap the hamburger menu (three horizontal lines) in the upper left, and select 'My apps & games'. Tap on the 'Library' column to see the history of what apps are and have been installed on your phone. You can sort the listing by 'Recent' or 'Alphabetical' so tap Recent and see what's been installed lately to see if there's something there you know you didn't add.
As for Clean Master, I agree with what
@psionandy stated. It's the equivalent to snake oil with a touch of smoke-and-mirrors. Its primary purpose is to give Cheetah Mobile, located in Beijing, and easy way to collect data about you, and it does so deceptively by re-enforcing a misconception that by wiping things like app caches will optimize your phone's performance. (In reality, this only makes your phone work that much harder as apps won't function without their caches so each time you delete its cache, when you start an app it has to rebuild its cache.) Go into your Settings >> Application manager, find the Clean Master app, open it, tap the 'Force stop' button, then the 'Clear data' button, and then the 'Uninstall' button.