Thank you for your query.
A number of causes come to mind. A definite reason can only be given after a clinical examination. Some of the diagnosis can be:
1. Dermoid cyst
(may be bony hard)
(usually not hard)
3. Bony growth
The size, exact location, mobility, fixation with deeper structures, etc. will guide in a clinical diagnosis. Otherwise we shall have to wait for the ultrasound
I hope I have answered your query to your satisfaction.
Wishing your granddaughter an early recovery.
Rajiv K Khandelwal