If you only create one user account they will have to know the password to log on. Another thing to do would be to restart in safe mode and go under the hidden administrator account. From there do the same thing. That way if your kids know about safe mode and they try to create an account for themselves they wont be able to log on to either admin account. The hidden one or the one you created for yourself.
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Phone: LG Optimus G Pro
Running: Stock JB from LG with Nova Launcher