My 9 year old daughter has yellow teeth. We brush twice daily. She has not had much exposure to antibiotics. Any ideas why? It bugs her a lot! Help :)

Could be natural. Teeth are all shaded to varying degrees. If your daughter truly doesn't like the color, perhaps her dentist can pumice them when she has a cleaning, rather than just polishing. This may be effective, if the cause is staining from the external environment. If the yellowing is intrinsic, then I would say when she is older, (16-18ish), perhaps a bleaching/whitening solution would help.
Wait to do anything. At the age of 9, there is very little that can be done. The color of the teeth at that age has already been established. It could have been from antibiotics when she was very young. Usually, even trying to bleach the teeth (which I would not recommend) at that age will not be too successful. The best solution is waiting until she gets older and then discussing treatments with your dentist.