What does the kid's joke at the end of Chasing Happyness mean?

This is the answer I found: that's what the director said when he heard the little actor joking with the actor who played his father during the filming. The director thinks this sentence can reflect the actor himself, so it is added to the script: The following is a reference.

Musino appreciates that kind of relationship, just like the natural and deep feelings shared by the Smiths and their sons in the next play, and he is determined to capture it with the lens. "The best example is the' knock knock' joke in the movie. This sentence was not in the original script. Then one day, during rehearsal, I heard Jaden tell a joke about knocking at the door. I think it should be good to put that relaxed feeling in the film, and it can also reflect some of the children's situation. Although he didn't have a good time with his father, he told a joke knocking at the door with a smile, which showed that he was not hurt and proved that his father was protecting him. The movie even ends with a knock-on joke. "

This is my own feeling after reading it:

Knock, the door rang, but there was no one. I think it's happiness. Happiness can't be seen, but it knocks at the door and inadvertently slips into the life of father and son.