Sally is the strong, silent type. Since she doesn't talk, her facial expressions are the biggest clues we have to how she feels about the whole fiasco. And you know what? That facial expression doesn't change too much throughout the story. It's mostly an "is this really happening?" kind of look.
Because of all that anxiety, we're super relieved when we finally see this little girl smile. What brings it on? Her brother's capture of the Things and the Cat's return to clean up the mess. Like her brother, she's had a big day. We're pretty sure the anxiety of the whole situation has gotten the best of these two kids.
Don't forget: Sally was the name of the younger third of the Dick and Jane trio. By not giving her any lines of dialogue, Seuss might be commenting on the ineffectiveness of those reading primers. Check out "Writing Style" for more deets on the Seuss v. Dick and Jane controversy.