It looks like a nevus (birthmark/mole) which is due to collection of cells deep inside skin layers. Creams don’t penetrate upto that layer so won’t help. She might need laser treatment for her condition but should go for it once body has fully grown (post 18years of age usually).
Next Steps
It would grow as the size of body grows as has been told by your doctor.
Health Tips
If the size or colour changes abruptly, show to a dermatologist.