I want to ask my girlfriend's parents to help me pay some damages that I caused to her car
What did you do to the car?
I crashed into a small tree while parking.
If you did the damage you should pay for it. You shouldn't expect them to want to pay for it.
I want them to pay for it because I want their money.
You have an obligation to pay for the damages that you have caused. You cannot rely on anyone else's money but you're own.