If she's the Jr/Child's age for the buffet's then she is-- I wouldn't worry about people asking for proof. I figure if you aren't lying, then you have nothing to hide. I also, think when you aren't lying you are confident in your answer and nobody questions it anyhow.
We recently went to a drive-in where under 12 was free. DS is really only 11. Anyhow, when they asked how old he was--- I stumbled and stuttered. I'm sure they thought I was fudging his age-- which I truly wasn't. Anyhow, they didn't ask for proof either. Not many people carry kids birth certificates with them.