Have to work with a person I hate?

Okay so I have this babysitting activity tonight. I didn't know anything about it until my mom told me about 2 maybe 3 hours ago. I have no problem with babysitting. But the girl I'm paired up with is a total Snob and stuck up little brat. She gives these little looks to people that makes me want to slap her. I'm normally a really laid back person. I hardly have problems with people and if I do, then I keep it to myself. I've never disliked anybody as much as her. My mom won't let me back out and i can't switch partners. And its not like I can just ignore her for 3 hours! I mean, If she says anything rude to me, or gives me one of those looks i'm scared i'm going to snap and tell her off. In front of like 1-2 year old's. I REALLY DON'T WANT TO! How do I go about this?