The menu did not mention if its 9 or what, but she really likes "plain" steak but is not a big eater and I hate to ask when we are there-would this be acceptable or rude.
Never be afraid to ask. They most definately should let her order it. If they didn't, I personally would leave, but I am sure you won't have a problem.
My DD is 11 and we have never had a problem with her ordering off of a childs menu ANYWHERE. Many times the person who seats us asks if we want a childs menu. It should never be a problem.
On our recent trip, with the dining plan, and a 10 year old considered an adult on it ... we often got her steak from the adult menu but they would prepare it for her plain when we asked. So that is also an option.