I am underweight and I am six pounds less than your daughter but her height there is absolutely no need to be worried about her, I understand your concern but she is right on a healthy spot. Once she hits puberty she might get a bit bigger though.
P.S. I've tried to gain weight. I really have. No matter how much I eat, I don't get fatter.... :(
don't worry about weight girl!
MAY THE FORCE BE WITH you!