I'm missin' a bit of information here. What's the length of time since you've been separated, if you're not already divorced? I'm not up to date on your situation, so that being the caveat, take my thoughts with a grain of salt...

It might be weird, but it might not. Is the child that is upset about it still hanging onto hopes that mom and dad are going to get back together? If so, then perhaps THAT is the issue that needs to be addressed, not that Dad and the GF and kids are coming to the play.

Next question is why is it a problem for multiple people who care about this child to be there in support of her performance? If it were a first date type of thing, then yeah, the GF shouldn't be there with her kiddos. If they've been involved for a while, then I'm not sure I understand the problem in its entirety.

My dad was married to the poster child for "evil stepmother." He couldn't go anywhere without her. Maybe that's the case here, too; however, the downside to that is that my dad didn't come to some of my events because she wouldn't attend with him. That being said, I'd rather see a woman on my X's arm who genuinely cares about my kid than one who is so jealous of anyone else in the picture (including his kids) that it would hinder him from coming at all.

Please don't take this to sound harsh, but it makes me wonder who has the issue with acceptance here? Is it the child, or is it you? Or are HER perceived issues actually her mirroring how you feel about him and the GF?

No matter what the answers to any of my questions/thoughts on the matter, don't ever discount how your emotional state is reflected in your child's behavior and desires....