I've had difficulty with my feet because of a condition called Morton's Toe. I have callouses and when I was a child much pain in my feet and legs. You can get info on that from the Internet by Googling that term.
My granddaughter has callouses because her ankles turn in causing her foots arch to rub against her shoes. She has them on both feet. Her mother has finally agreed to let me take her to a podiatrist for an evaluation.
I can't adequately answer that question unless I know where the callous is. Is it on a place that her shoes rub? For example on the outer or inner edge of the ball of her foot. If so, the shoes don't fit properly. They may be too small or too large in width. A callous frequently does not cause pain but there is frequently pain before the callous forms. Whether or not she needs different shoes feels uncertain to me. I'd be sure to check for proper fit when buying her new shoes but suggest that it might be OK to wait until she outgrows these if her feet are growing.
If it's under the ball of her foot it may mean that she has a condition related to Morton's Toe. If she doesn't complain of pain then you' wouldn't need to do anything about it except be aware that if she has pain in her foot or legs it would help to have her see a podiatrist.
If the callous is on her heel, it may be that her foot slides too much in her shoes.
Or the callous may be caused by something she does repeatedly when she's not wearing her shoes.
You don't need to put anything on it if it's not causing pain. Lotion, cream won't take away the callous.