They will work fine on 12V. An Amp is an Amp, is an Amp. It is defined as a really large number (6.24150948×10¹⁸ ) of electrons flowing past a boundary every second.

The voltage rating on the breaker is how many volts it will sustain turning off without an arc across the points continuing to carry current. 30V works fine for a 12V system as does 50V. The interrupt rating means that even a large surge like 3000 Amps won't weld the breaker points together and cause the breaker to not open. If you hit it with something like 6,000 Amps, it may weld before it could open.