You can probably ask for them to bring the child food - but you will have to ask, they won't do it automatically if nobody at the table is under 10. The 12 year old will still pay the adult price.
As mentioned, Disney's age cutoff for the adult/child deliniation at its restaurants is under 10.
Yes, just ask and they will bring her children's food if she wishes. We were there with a mix of children and adults. When the server asked if we wanted any more, my sister (an adult) asked for more chicken fingers -- she just loved them and ate from the kids' platter.
Yeah, overall, the food wasn't good. The restaurant was pretty neat in that it rotated and we had the characters when we went, but we aren't going again.