I had my fingers slammed in a door a few times as a kid. The first time I was little and it was a car door.
Turn the clock back about 8 or 9 years. The same son was with me at a store. We go into the parking lot. I open the trunk to put my purchases away. He is standing next to me. I go to close the trunk. But before I swing it shut, I look down to insure his fingers were out of hte way. As I am in full swing of shutting the trunk, no time to stop it. He inserts his fingers in the way and they get slammed into the trunk like in your situation.