My 2 sons were both diagnosed with hydronephrosis ( enlarged kidneys) in utero...via ultrasound...one was ultimately fine, the other had ongoing ultrasounds for 4 years after birth and surgery at 6 to correct what they thought was a blockage in the urethra. (Turns out it wasn't as dramatic as that..long story for someone who wants to hear it!)....but that being said, it is a common thing in boys. An infection is a tip-off if they hadn't picked it up before ( like on an ultrasound). For a girl? All my research concerned boys...so my guess is tests and an ultrasound would find the cause for enlargement. Some involved an IV to increase fluids, then they do what I called a "live" x-ray to watch the kidneys drain, measure how long it takes, etc. While IVs are no fun, in a hospital setting the technicians we had were so good at it ..he suffered little. I would tell you, if she's on antibiotics for an infection, I'd up the probiotics to fight yeast. Plenty of links here on WTM about antibiotics and immune systems and candida. There ARE natural things to do for kidney stones if that should be it.