By movie's end, Mr. Banks has lost his job and come away a better man.
Fortunately, he feels great—and not at all like jumping into a river (which is what the maid, Ellen, thinks he's done). He's ready to get that "spoonful of sugar" and mix it into his family's life. He gets the kids to go fly a kite with him, while Mrs. Banks comes along too—using her "Votes for Women" sash to make the kite's tail (see our symbol section for more on this).
They head down to the park, all singing the tune, "Let's Go Fly a Kite":
Let's go fly a kite
Up to the highest height
Let's go fly a kite and send it soaring
Up through the atmosphere
Up where the air is clear
Oh, let's go fly a kite
Everyone has a great time, and Dawes Jr. arrives at the park to tell George that his father's dying wish was to re-hire Banks as a partner in the bank. George gratefully accepts—now, he'll probably be able to handle it without getting all discipline-obsessed again.
Meanwhile, the wind has changed and Mary Poppins needs to leave. Plus, it's emotionally the right time—the Banks parents are connecting with their kids, meaning that Mary has accomplished her mission.
Mary says she's not upset to be leaving, because:
MARY: Perfectly practical people never permit sentiment to muddle their thinking.
To which the talking parrot-head-shaped handle of her umbrella says:
UMBRELLA: Well, I'll tell you one thing Mary Poppins, you don't fool me a bit.
Of course, Mary loves the kids. But she doesn't need to act as a substitute parent for them because the Banks kids now have two perfectly good parents. She's able to launch off on her umbrella and go back to…wherever she came from.