Oh boy.
I've been through a terribly bitter divorce. I've been to mediation about 800 times over, I hate to say...stupid, things like this.
Parents have to be respectful and communicate. They have to be willing to trade or switch weekends. The older kids get, the more important that is.
We were told it's not about Mom's time or Dad's time....it's the kid's time. And, if we couldn't work it out, then the judge would step in and decide and we'd basically have no say. So, did we want to work it out or did we want the court to make all the decisions?
My ex and I actually communicated everything in writing and kept copies so that neither could say they weren't informed or someone did something at the last minute.
You may need to write your ex a letter and let him know that you are asking for another mediation session to deal with issues such as this as your daughter is getting older and she should not be punished because you two can't agree or be flexible in order for her to do things that are important to her.
Courts don't like kids missing out on things because parents are jockeying for position.
Put it in writing, in a non threatening way, that if your ex does not want to attend the event, will not trade a weekend with you, AND will not let your daughter participate in something she's looked forward to, you feel that going back to mediation to address these issues is the only option for the well being of your daughter who has activities and social interests of her own.
Keep a copy of it. Keep a copy of the postmark.
Show you are trying to reach a compromise.
In California, kids this age are allowed to be talked to in mediation. Let your husband risk her telling them that this was about punishing you or teaching you a lesson. He'll end up with less visitation than he already has if he's not careful.
Why can't he take her to the event? It's his weekend. Why does she have to miss it?
She will grow to be more distant from her father over these things. That might not be the outcome he expects, but it will happen. He can blame or punish anyone he wants, but if he cared about her, he'd let her go.
Just my opinion.