Admin should be you, then, you could set accounts for each child, or just have a general account for all of them, like the "guest user" account with parental controls. Then enable "fast user switching".
Whenever you are finished and leave the computer, all you have to do is click on your account name in the top right of the menu bar and select the guest account. That is the easiest way to transition.
When you come back to the computer, simply click on the "Guest user", again in the top right of the menu bar and select your account name. It will prompt you for the password, then voila...
Side Note:
I leave my computer on 24/7 and never shut it off. When not in use, it goes into sleep mode, and awakens at a preset hour in the moring just in time for the alarm I set to go off, or whenever I tap on the keys to use it again.
It's not for everyone, but I do not have time to wait for my computer to turn on from shutdown, time is of the essence...