Mary Poppins is a pure G-rated experience. No one's getting murdered. No criminals get cut down in a hail of bullets. And there's no "adult" content—beyond visiting a bank.
Everything is fun, light-hearted and whimsical. The world of Mary P. is for kids.
The only really objectionable thing in the movie is Admiral Boom's unpleasant racism—firing his cannon on chimney sweeps because he thinks they're African natives. It's a totally offensive and badly dated moment.