Well like I said before, it's very sweet that you are concerned about your girls comfort....
Just have her pack light and if her back does start to bother her, I'm sure you will step up to the plate and help her out.
She should be able to carry a backpack just fine though...if it is a proper fitting pack and packed well.
"The World is a book, and those who do not travel read only a page." ~St. Augustine"
Come visit me in the DR!