This seems to be a case of Exostosis
.....also known as bony outgrowth. Chest is not an uncommon site.
This is a dangerous area to have it as it is just above the heart.
Most of these are benign and not to worry about until they start causing symptoms like rapid growth or pain or infection.
Since your daughters lesion is symptomatic I would suggest you get it removed surgically.
But be sure to consult a good orthopaedic surgeon
who is well versed
with operating at this area .
Also check for any such similar outgrowths in other parts of the body.