Can I legally tell Grandma to BUTT OUT

My son’s father (not my ex, we were never married) and I went through mediation 2 years ago. When we did this mediation his mother and lawyer were present and he never spoke a word and let his mother make all the decisions. We now have week on week off custody and it is not working, she is the only one taking care of our son not him. It is time to revisit our mediation and I have had multiple school related issues (like dropping him off early and late un supervised) causing me to the request the change in custody. Do I have a legal right to refuse his mother’s access to this mediation meeting?

No. Unfortunately, the grandmother can go. Either side can bring whomever they wish to the mediation.