Hi,
I wanted to get the same results. Turns out the 'Master password' is like a master key, it'll open any label password.
So to get the effect you're after all you need to do is under each label set a random password, then set the master password, don't worry about remembering the random password as the master password will get you in but keep the kids out!
If you want an option accessible to the kids (boot from local hard drive) then just don't set a label password on it.
Setting passwords doesn't affect FOG's ability to boot tasks set by the web interface.
HTH,
Seb