Can you talk to her mom and ask her if she will be home that night? I have been in this situation and that's what I did. I didn't have any other choice, except staying at home. So I called the mom and said that I liked the babysitter, but was worried because I was going into the city. Then I asked if she was going to be home in case something happened. I already kind of knew her and it wasn't awkward to ask. The mom was totally understanding and said she was going to be around and to have fun.
If it's a night time wedding, the kids will be sleeping part of the time you are gone anyway.
If it's a daytime wedding, maybe you could set up some playdates and have the sitter later.
I have a son who is in 6th grade and he is more responsible than my 8th grader! So you really can't judge by the sitter's age.
M.