If kids know your PIN, they can use their devices any time (for free) or they can edit settings.
So please set a hard PIN !
Note if they try to guess your pin incorrectly, after three failures it will add delays that keep multiplying after each error eg 1 minute, 2 mins, 4 mins
If you "suspect" they have been up to no good you can investigate!
Parents can look at session history to see if their kids have used parent accounts on their devices. (then you know)
If your kids know your own phone's unlock code they can access ScreenCoach app on your phone, if you want to block this, You can set up your device to be a shared device, so it will log you out of ScreenCoach after screen lock for more than X mins.
However, this means each time you access ScreenCoach you will need to enter your PIN.