Try this:
When confronted with the blank screen try pressing the Windows Key+L to get to the logon screen. Once at the logon screen, if the kids' account is still showing as logged in then logon to your account, open up Task Manager, click on the Users tab and log other logged on users off from there.
As for switching the PC off safely in situations like this, hold in the PCs power button in for a few seconds to shut down.