When Shirley is out with Joy giving their dolls buggy rides and Joy wants to operate on Shirley's doll, Shirley says she doesn't want Mary Lou to be operated on. But the doll she actually has is not the small one she named Mary Lou, it's the larger one named Lupee given to her by the aviators.
When Joy offers to give Shirley's doll an operation, Shirley picks her up and calls her doll Mary Lou. This is the name Shirley gave to the doll her mother gave her for Christmas, but the doll she has now is the one the aviators gave her, named Loopy. You can tell because this doll is larger and is wearing an aviator hat.
During the song "On the Good Ship Lollipop" while the plane is taxiing you can see the edge of the screen where they are projecting the outside. This is most obvious when she sings the line "...and there you are..." just after "Cracker Jack Bands fill the air" the first time.