I have two barns on the same electrical circuit. With both barns together there are five receptacles and about 5 or 6 lights. The first barn is wired with primarily 10 AWG wire, and the second (fed from the first) is wired with 12 AWG.I would like to install some sort of "switch" that would allow me to cut all power to the second barn from the first. Can I just install a 20 AMP light switch or is this a bad idea? Should I change the breaker from 30 amp to 20 or can I do that? What should I do?

Any separate building fed by another always needs some sort of disconnect.

A simple 20 amp switch will work fine as a disconnecting means for one circuit.

Yes but since the circuit is on a 30 AMP breaker could I (in theory--although I doubt it would happen) overload the switch?

I completely missed where you mention a 30 amp breaker, due to the #12 wire in the circuit, yes, it needs to be changed to a 20 amp.