Usually kidney pain is in the upper mid back area where the kidneys are located not in the lower back.
How low is the back pain? It could be she has coughed so much she has pulled or strained muscles in her back. Also, your lower lobes of the lungs are kind of low on your back and since she has pneumonia that would cause that too.
Ask her doctor though to be sure. Poor thing. I have had pneumonia and it is horrible.
I don't know if this is kidney infection. I had this when I was young, and I was hospitalized. Is your daughter in the hospital? If so, doctors shoud be able to determine this. Blessings - Blu