Dear M.,
These types of calluses are most likely caused by wearing ill-fitting shoes. I'm a shoe fiend, myself, but 7 is awfully young to sacrifice foot health for cute shoes.
I would have her soak her feet in a pan of very warm water and epsom salts. That will help dissolve the callus and you can use a dry wash rag to rub the layers of skin off. (When I broke my leg, I did this to get rid of the yuckiness that had built up under my cast and it really worked. Plus, the soaking feels really nice).
They even have non-medicated pads that you can put on her little toes to protect them from rubbing on the inside of her shoes. Rubbing Vick's Vapo-rub or vaseline on her toes and having her wear socks to bed at night after soaking will also help. Learning to take care of her feet now will hopefully become a habit that will stick with her so she doesn't have problems as she gets older.
Best of wishes!