What do I do about my friend's bratty little cousin? advice?

Tomorrow I'm going to Water Park of America with my friend and her cousins. One of her cousins is a 6 year old bratt. Whenever she see's me she'd be like "oh here comes the witch" and giggle. Now, I don't let that bother me at all, but when no one is looking, she will kick me (with hard little shoes) and I'll usually come home with a big colorful bruise. I've ignored it before but is there anything else I could do? I don't really want to bother my friend because she was nice enough to invite me...