I clearly remember my Mum taking me to the dr when I was about your DD's age with exactly the same thing. I had the hard lump under one nipple only and Mum must have freaked out.
It was all fine and just the beginning of my lovely lady lumps developing and very its very common for them to develop one at a time.

For what its worth it was my left breast and that is the bigger of the two to this day. Dunno if thats just a coinky dink or not

If you are concerned I would definately take her to see your Dr if nothing else to put your mind at rest