I brought my mom with me to my first interview (during my first application cycle). They were very accomodating of guests, and most of the people that interviewed that day had at least one of their parents with them.
All other interviews I went by myself. I liked it a lot better by myself, because I could focus on me. When my mom was with me, I was worried about the impression that she would make (not that she would make a bad one but it's just one of those things that runs through your mind when you're already stressed). Another thing that I really disliked was that the parents seemed to talk to each other about their kid's qualifications. It was like a competition, whose kid is better? They were trying to size up the other applicants there that day. My mom was very good and didn't divulge any information that I wouldn't have wanted her to. Instead of trying to one-up the other mothers, she would just say "oh that's nice/great/exciting" when someone would state that their child had won some award or been #1 in their class.
I don't know if it would be that way everywhere, but I was super uncomfortable when I was touring the school and I could hear the parents all talking about how their child must be a better candidate than everyone else's children.