J.,
I completely agree with the other moms. I was divorced when my son was 3 and my ex had limited visitation for many reasons but at one point we used a mediator and it was horrible. Everyone goes into the mediation with a goal in mind, you want something, your ex wants something and the mediator's has his/her own goal to reach an agreement in the room, no matter if that's a resonable agreement or not. The goal of the mediator does not have your child's best interest's at heart and will not be advocating for her. My very strong advice is to get a lawyer and get one now.
The moms who mentioned that once it's in writing it's tough to change it are also dead on. Your ex may fight you every inch of the way and his wife will be there cheering him on. I would also make sure that it's clear in your agreement that the visitaion is with her father, not his wife. If she's there for a month and he's off at work, guess who she's spending all her days with?
Mom, you are the buffer between your daughter and the rest of the world, stick with your mother's intuition, advocate for your daughter and surround yourself with patient, understanding and supportive people. You can make it through this. If you need resources, email me and I'll see what I can help you find.
By the way, my son is now 14, our custody agreement works and my son is a happy, healthy and successful young man.