I think this moment symbolises the process of the Disneyfication of P.L. Travers’ Mary Poppins.
Mrs. Travers always clearly stated what she wanted. She didn’t want Mary Poppins to sing, she didn’t want her to be whimsical and she didn’t want the story to be too sentimental.
But he did his own thing anyway and ignored all of her requests. Just like in this moment.
Disney’s Mary is almost the exact opposite of P.L. Travers’ Mary. Walt basically added far too much sugar to her Mary Poppins.
When she merely wanted a spoonful.
And nothing more.