My daughter drove my sons car with the parking brake on - it doesnt seem to work now 
What is the cost to replace and is there anything else that would be damaged. Thanks
IF the pads just got burned out, then the cost will be the same as a rear brake job. If there is more damage, the price goes up. There’s no way to tell until the wheel is pulled, and the rear brake system, including parking brake, is evaluated.
A short list of parts to worry about: Brake shoes, drums, cable, linkage plates, and brake springs if a rear drum set-up or rotor-drum set-up. Rear disc brakes, I’d worry about rotors, pads, caliper, and parking brake cable.
it doesnt seem to work now
Could you be a little more descriptive?
The drums in the back probably just need adjustment. But the shoes may be worn to the point of needing replacement.