After the fall, if there had been no ear/nose bleed or clear fluid draining from nose or fits
or altered consciousness or vomiting- there is nothing much to worry.
The bump will get okay in about a week.
Any minor bony injury
to the skull may take about 3 months to heal.
If the dent persists and you have any cosmetic issues over it, you may consult a doctor for the same.
Hope my answer helps you.
Wish your kid good health.