[ralfd]

Control-Click. I don't think it is an oversight, because "Manual Override" is touted officially as a feature:

http://www.apple.com/osx/whats-new/features.html#gatekeeper

And even explicitly mentioned in a system dialogue:

http://cdn.arstechnica.net/wp-content/uploads/2012/07/gateke...

I am still not on Mountain Lion, so I don't know if you need to enter a password (which children shouldn't have) or right clicking is enough?