I can actually believe that you don't know what they're raising money for. I'll bet you lost the paper that came home. It would be better if you made a folder for this kind of stuff so that you can keep up better with what your child is doing at school. You're just at the beginning of many years of school and you need to know this kind of stuff. Most teachers have the kids put the notices from the school in their backpack. The first thing you're child should do when she gets home, after going to the bathroom, is empty her backpack with you so that you can see what all is in it.
If I were you, I'd go to the principal and tell him or her what you have been told, and that you are having a hard time with this. Tell the principal that you want to sit in on the rehearsal and hear what the music teacher is talking about. If the principal says no, tell her that when you call the news and tell them about this, that probably a reporter will want to interview both you and the principal to ask why your daughter couldn't participate. I will bet if the principal thinks you'll call the news people, she will re-think this.
If what they mean by "not childlike" is that your daughter can't carry a tune, and if she sings notes that are wrong, all over the place, different than the other children, then they should have come to you and talked to you about that in honest terms, rather than just calling it "not child like". If that is the case, it is hard for the audience to listen to one child who is not singing the right notes, especially if she doesn't know it.
Not everyone can carry a tune. Some people are truly tone deaf. Can you sing in tune? Do you KNOW what it means to sing in tune? Can you tell if you or your child sings "wrong" when you sing with the radio?
If your daughter can sing the right notes, but her VOICE sounds different from the other kids, then perhaps it is a BLENDING problem. Is your school's music teacher new at her job? She may not know how to blend children's voices together. That's not your daughter's fault. It is up to the teacher to blend the voices together. She should have worked with your daughter to do that from the time she started singing in the choir.
Whether it's that her voice doesn't blend or she doesn't carry a tune, there should be a solution to this problem, OTHER than telling her that she cannot participate. The teacher could find something else for your child to do. She could turn the pages for the pianist. She could hold the music, hold the teacher's clipboard, be her "assistant". Anything other than being made to stay behind. The biggest problem is that the school is being PUNITIVE instead of trying to figure out a solution for the problem. And they haven't approached you in a way that even makes sense.
As a musician myself, it pains me to see this approach. All it does is take away an innate love of music that all children have. If a teacher doesn't know how to deal with a child's lack of singing talent or ability, the answer is NOT to shun the child. The answer is to find out how to help them. My guess is that the teacher doesn't know what to do. Leaving your daughter out is not the right thing for her to do.
Go to the principal now and tell him or her that your daughter's entire school career and music is on the line right now because of this unfair announcement of the teacher's, and you want a different solution to the problem. Print out what I've said and give it to the principal if you want. I substitute teach in schools and have worked with many FINE music teachers who would never do this. They work with the child and never shame them for their inability to sing perfectly. And that's what your daughter's teacher should be doing.