you can do as you please. She smiled, Yes you do what you please first then I can do what I please—
The speaker finally convinces his grandmother to go to the hospital. She agrees to do what he wants, as long as she can do what she wants later.
You could definitely take these lines at face value and say that the grandmother truly believes that she'll live long enough to do what she wants again. Or you can see them as sarcastic: "Oh sure, I only get to do what I want after you get to do what you want, your majesty."
In either case, she's resigned to going, and she knows she'll probably never return home. All the same, she decides to play her part in her grandson's optimistic play.
Notice the repetition—three times—of the word "please" in this stanza. That seems odd for such a short stanza. Did Williams run out of words here? We doubt it.
Instead, it's almost like there's a kind of begging for life in the subtext of the dialogue. The speaker and his grandmother are both talking like it's a done deal that she'll be okay, but underneath they're saying, "Please, please, please... don't let her die."