I used to crack my knuckles, bad....like all the time. Until i started experiencing pains in my fingers and hands....im in my twenties by the way.....i went to a doc for the pain and started unconsciously cracking my knuckles. The doc outrightly told me....do not crack your knuckles. you can develop arthritis from it.
Now my knuckles are bigger, because of the cracking, and i experience pains in both hands and fingers and sometimes mild swelling of the fingers and knuckles.....im supposed to get tested for arthritis.
My advice to you is to try and get your daughter to stop cracking her knuckles. i actually started around the same age as her, and if you can prevent this type of discomfort in her, then by all means, go ahead.
if you want even more proof, i find that doing a simple search on the net helps.