Hi S.---First, 100% juice IS a sugary beverage, by definition. It just is natural sugars. Milk has a lot of sugar in it also. I would definitely cut back on those two, especially cows milk. Go to www.strongbones.org to learn why milk is not the healthy food that we are all led to believe.
Has your daughter been losing weight? This is what happened to my neice when she was 8 yrs old. She and her sister had gotten some sort of virus and within 2 months she was diagnosed with type 1 diabetes with no family history. My sister was annoyed with all of the thirst and trips to the bathroom until friends, who hadn't seen Meggie in a while, said she looked like she had lost weight. I hate to say it, but I would definitely have another doc check on the possibility of type 1 diabetes. The symptoms are classic. As others have alluded, docs know a lot but they don't know or recognize everything. AND, tests are not always 100% accurate.
Another curious thing for me is the comment that her urine had good concentration for all of the liquids she is consuming. If she is drinking that much, I would think that her urine should be dilute. That doesn't make sense to me. Have her checked again, or get another opinion.
The body is a very amazing machine. Until you get this figured out, please support her body with an optimal diet (which is good for the entire family). Be sure you are consuming mostly plants, fresh fruits and veggies, whole grains, legumes (beans), nuts and seeds (but not too much of those...contain lots of fat). Minimize animal protein, as too much, more than 10% of daily calories, promotes diseases like cancer, heart disease and diabetes. Please read The China Study by T Colin Campbell to find out why too much animal protein is not good for humans. I know it sounds crazy, but the research is clear.
If she is having immune system issues, this diet will help to balance and strengthen it. I would also recommend a high quality food based supplement. I can share what my family does, if you are interested. This food supplement comes in a gummie form and contains 17 different concentrated fruit and veggie powders and is backed by 12 independent studies, several of which show immune system support. There is nothing like it on the market.
Good luck. Saying my prayers. In health, D.