Controlling output voltage using Arduino
You should first check the rating of heat pad , how much current is required or how much power is required 

so you can calculate by P=V.I

that how much current is required now check that how much current micro controller can give 

if its not sufficient than you have to connect second source for heat pad and use micro controller for switching 

relays are good for high voltages like 220 or 240 but for 5 to 12 v relay is too much

you can use just transistor circuit for switching those 5 to 12v