How do I politely ask her to stop being dirty?

My girlfriend keeps making my car dirty.

i’m driving a rental for the time being, I keep it as clean as possible because I deliver food, and I have to return my rental at some point. I drive a decent sized SUV.

She's been taking her kids to football practice in my car, The kids have dirt on their shoes, they’ve stained my floors, & They’ve rubbed their feet on the back of my seats which also causes stains. There’s grass and dirt everywhere.

I don’t mind her using it, but my car HAS to stay spotless since I deliver food.
it’s not an option.

How do I tell her this in the most polite manner?

You have to walk on eggshells with my girlfriend. I’m pretty sure she’s gonna get mad since I’m not really a big fan of kids in the first place, she thinks I don’t like her kids..

You have to walk on egg shells with her sometimes.

How do I politely ask her to stop being dirty?
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