As someone who was in her first real bra at 11 I can understand how awful it is for her. Having said that I also know the importance of wearing the right bra. I was an E cup by the time I was in year 8 and am a G cup now.
I would buy some cheap ones from kmart in a couple of different sizes. I would say either a A or a B and go from there. They have some cute bras there. Work on her t-shirt size and see how you go. You can always take them back if they are the wrong size.
Let her try them on by herself and tell her to let you know what shes comfortable in. Don't make a big thing about it. Its just part of life.