Thank you for your query,
This bump is due to a contusion
formed that time because of accumulation of blood in the scalp
layers forming a hematoma
Gradually the blood inside get disintigrated and get absorbed into the blood leaving no swelling.
But some time it get organized forming fluid or get calcified with a hard bump feeling.
Consult your Pediatrician
to know any intervention is required.
Hope I have answered your query.