Our apartment has what appears to be a NEMA 6-20 socket in the living room (used for the air conditioner).
Right now the circuit breaker is off but in the summer we'd like to use it.
I can't find any child proof covers or guards. Any suggestions?
It's not a perfect solution, but you should be able to use a pair of snips to modify an existing 5-20 cover to fit in a 6-20. Simply cut off or cut down the blade so the cover doesn't actually slide into the horizontal slot of the receptacle. If you wanted to keep the blade, you could always cut it, rotate and then superglue or epoxy in place so that it fits.
A child-safe cover may not be easy to find, but lockable outlet covers for electrical outlets are widely used in industrial and outdoor environments.
Here's an example of a watertight outlet cover which has a cover that can be fastened shut with with a zip tie, or even locked with a small padlock if desired.
Also, in many cases such an outlet is on its own circuit. If so, turning off the circuit breaker will render the outlet harmless even if you don't cover it.