When using Parental controls, what's the best way to manage time limits?

When using Parental controls in Mountain Lion, is there a way to my my teen to pause or lock her session without logging off? It seem like this would be an essential feature, if one would get called away from the computer? I would think it's fair say that having to log off & lose you place in a game or session would be disruptive. If there were a lock or pause button, it's obvious that my teen wouldn't/couldn't be using the computer during that time & wouldn't be fair to count that against her!! If there's not a way to do this in Mac OSX, is there a third party solution?