What is fair?

My soon to be ex dropped a bomb on me when he picked our darling daughter up today for his time with her. He wants to take her to Palm Springs for Christmas.... What do I say to this? No way in fucking Hell? I mean she is his little girl too. But it's like the 4 year old inside of me is screaming "I DON'T WANNA" And I don't. I mean seriously who wants to be separated from their kid on Christmas which makes me a bag for saying no because of course he wants to have his little girl with him for Christmas. And no, before anyone asks, we have no formal child custody agreement in place because I'm a fuck wit and left with out getting one because I needed to get out... So I have to find a balance with what I can handle and what is fair.... God, I hate being a grown up....