You definitely need to get the kittens moved. I don't know how the laws are where you are, but here, if anyone is hurt on your property, you are responsible whether or not they had your permission to be there.

Not only that, but I'm sure you don't want the little girl to be hurt in the first place. Move the babies (not far, and make sure the mommy knows you're doing so. If a cat can't find her kittens for a certain period of time, she loses her mothering instinct) and seal off the car. You might need some repellent to keep her from moving back into the car again. She'll probably move the babies, but as long as they aren't in that awful car, you should be okay.