What to do when kid hits head on door?

Assuming that no visible wounds are there, just watch the kid over the day for signs of head injury. They include:

-Sudden drowsiness (taking naps when they usually don't)

-Eyes with different sized pupils

-Slurred speech

-Sleeping soundly but not being able to awake

Those are just a few of the most common signs. As long as the kid wasn't running at a door with full force while it opened, there shouldn't be any major trauma.