I would be honest with her. I know it will be very difficult. I am somewhat shy when it comes to face to face. I'd do it on the phone. Maybe simply explain, "I'm glad we're friends. But there is something that is really bothering me and I don't want you to get upset with me." Then I'd go into specifics. It was YOUR house they were in. In my opinion, that's where kids should behave the BEST, when they're at someone else's house. Was the mother there w/ them? If she wasn't, she probably doesn't know. I'd explain their actions to her. If she was there and did nothing, that's another story.
In either situation, I'd be completely honest with her. (I'm a chicken, though, I'd do it over the phone.)
Best of luck to you!
~K.