Is it that they want to go out separately for her birthday, maybe a special dinner, just the two of them? That wouldn't be an unreasonable request.
I also understsnd not wanting a combined party. Everyone gets blended in and glossed over, but after 25 or so, adult birthdays aren't the same as they used to be. She should be adult enough to go with the flow. Sounds like she's very young.
Have a combined party, then let them celebrate hers, then the child's however they want, separately. 4951