I remember when my niece was little (around 4 or 5) and she had redness around her vulva. It turned out that she was not wiping her self dry when she peed and her vulva became inflamed because the urine was irritating it.
Also, pee that is very concentrated can burn. Is it possible that your daughter is not drinking enough fluids and the phosphorous in the urine is causing the burn??? She should be drinking enough fluids during the day that the urine is a pale color, not dark yellow (unless she's had a multivitamin and then the b's in the multi will turn the urine a bright yellow)
D-mannose and cranberry juice are very helpful in preventing and even stopping uti's. Make sure the brand of cranberry juice has a high concentration of cranberries. My sister used to make "fresh cranberry juice" by using a package of cranberries in a pot covered with water, simmering until the berries had released the juice and then adding a can of frozen apple juice, without added sugar,[edit: strain and pour in pitcher] voila cranberry juice without preservatives and extra sugar.
Finally, I was prone to uti's when I was little. The doctor determined that my urethra was "too narrow". It was widened in surgery. I don't remember getting many uti's after the surgery. You might take your little girl to a urologist for an evaluation.