Thank you for your query,
The lump may be due to a hematoma
which is due to collection of blood by the injury.
Nothing to woory as it gone down.
Watch for signs of brain injury
like vomitings, headache, ENT bleeding, abnormal sensorium, fits, and decreased limb movements.
If you find any abnormality, consult emergency immediately.
For pain you can give Tylenol
or Advil in prescribed dose.
Consult your Pediatrician
to assess the extent of injury due to fall and its management.
Hope I have answered your query.