but is it a good choice for her problem?
It depends on the source of the problem. If she received poor nutrition, coupled with being caged constantly, and got no sunshine, then there's a chance good food, light exercise, and some time in the sun will do her well.
But if it's genetic, there's nothing you can do. And as for if it will hurt her, yes in the terms that arthritis will set in quickly and begin to deteriorate all those joints.
GSD's are the most common dogs we have to euthanize due to downed pasterns like those and severe HD. And sadly, I cannot even tell you how many we've done before they even turned 1 year of age.
Also, you've been given some good advice on "backing the train up" and start integrating her into the family the right way. This means slow introduction to the other dogs and to your child. Don't rush her into anything.
And I also have to say I was petrified when I saw that picture of the two dogs and your child. Not only would I not allow my child to between 2 dogs that don't know each other, but my child is never and will never be between 2 dogs period. It's a recipe for disaster and one your child will pay the price for.
And obedience lessons are a great idea. Not just for the actual obedience and training part, but because it's one of the best ways to actually bond with your dog and build up trust and understanding.
Let us know what the vet says and say responsible.