"Kramer vs. Kramer" (1979)
Here is a part of the plot summary from Wikipedia describing the scene where the boy falls on his face:
"Ted befriends his neighbor Margaret (Jane Alexander), who had initially counseled Joanna to leave Ted if she was that unhappy. Margaret is a fellow single parent, and she and Ted become kindred spirits. One day, as the two sit in the park watching their children play, Billy falls off the jungle gym, severely cutting his face. Ted sprints several blocks through oncoming traffic carrying Billy to the hospital, where he comforts his son during treatment."