Jane and Michael probably aren't going to end up writing angry tell-all biographies about their evil mom and dad…because Mr. and Mrs. Banks are both pretty sound, parenting-wise. But the credit goes to Mary Poppins.
Mary gets Mr. and Mrs. Banks (but, particularly, Mr. Banks) to see the light and start spending time with their kids, showing their love. The problem isn't that the Bankses don't love their kids: it's that they're so preoccupied with other things that they're failing to appreciate this time in the life of their family.
They need a good shock to switch things around, and Mary's the right person to administer that shock. She's sweet and nice, but she also understands that, only by shaking things up, will the Bankses become a cohesive family unit again.
The Banks family is a wee bit dysfunctional because the kids don't have enough freedom: their father and a series of bad nannies are always trying to control them.
The Banks family is a wee bit dysfunctional because no one's paying enough attention to the kids. The adults are all distracted with their own problems, and aren't taking the proper amount of care in selecting a nanny, or spending time with the children themselves.
Mr. Banks is stuck in a sad, lonely prison: the bank where he works. And the worst part is—he thinks he enjoys it.
The bank has colored everything else in his life, until he thinks that all you need to run a family are the same tools you need to run a bank (like discipline and efficiency). In other words, he's lost that lovin' feeling. In order to get his groove back, Banks needs Mary Poppins to help him.
Only after she helps get him fired (by suggesting that he take Jane and Michael to the bank) does he open up and start living the life that's always been waiting for him to live it.
Mr. Banks feels isolated because his work at the bank has shifted his worldview: he thinks life is all about the kind of discipline you need to manage money, instead of just enjoying life.
Mr. Banks feels isolated because of the British class system, which has forced him to act like a proper upper-class gentleman instead of letting him let his hair down and chill.
Even though Mary Poppins makes the kids happy through extreme methods—leading them into a cartoon world, for instance—the movie's actual message is that happiness doesn't cost too much.
In the end, Jane and Michael are just as happy flying a kite with their Dad as they were venturing into the animated English countryside with Mary or dancing to "Step in Time" with the chimney sweeps.
Where Mr. Banks initially thinks that Dawes Sr. and Jr. have the keys to happiness, Mary shows him how wrong he really is. She cheers up the entire household and gets him to follow suit.
According to Mary Poppins, the key to happiness is enjoying the small and simple things, like feeding the birds or flying a kite.
According to Mary Poppins, the key to happiness is always adding an element of fun to everything you do, even if the task at hand seems boring.
Mary Poppins blows minds for a living: she leads the kids into trippy animated worlds, uses magic to clean up the nursery, and has an uncle who floats when he laughs. Oh, and she flies.
Naturally, a lot of eyes are bugging at this—when Ellen, the maid, first sees Mary slide up the banister, to the top of the stairs, she almost freaks out. But Mary isn't just a show off: she's using shock and awe for a purpose: to liven people and shake them out of their boring everyday sense of life.
By the time she's done with them, they're able to find new enjoyment in everyday activities—like flying a kite.
Mary Poppins inspires awe in order to communicate wisdom and shock people into a greater awareness of themselves and what their situation in the world is.
The awe Mary Poppins inspires is actually inspired by the characters' own imaginations. That's why she denies that they ever participated in a horse race in a magical cartoon universe, when Michael reminds her that this happened—she's trying to make Michael look to his own imagination for amazement, not to magic.
Mary Poppins drops a lot of wisdom on Jane and Michael—and on Mr. Banks—throughout the movie. So does Bert.
How did the gain all this knowledge and wisdom? No one knows. Their backgrounds are enigmatic, obscure. But their wisdom is definitely better than the Dawes' version, since that just makes Mr. Banks totally unhappy, tense, and barely able to cope.
Mary's short sayings—like "A spoonful of sugar helps the medicine go down" and "Enough is as good as a feast"—make people relax and accept the lives their leading, instead of being constantly anxious about what's next or what needs to be under control.
The reason Mr. Banks is unhappy is because he spends time worrying about the past and the future. The wisdom of Mary Poppins and Bert allows them to live in the present, making them happy.
Mr. Banks is unhappy because he's not paying attention to the passage of time, and isn't realizing how fast his children's childhood is blazing by. Mary and Bert help him tune into this and enjoy the process of watching his kids grow up instead of being a strict disciplinarian.