I wonder if this may be particular to performances in the tour. I saw the piece on Broadway and NT Live and in neither instance did the moment you speak of elicit laughter. Nor was there any display of undergarments.
I (and I think most of the audience) thought the childish behavior that Beatrice referred to was the childlike way that Catherine jumped on Eddie and touched him physically. It was clear that Catherine was physically free with Eddie like a small child is with her father. It was also clear that neither Catherine nor Eddie was consciously aware of the sexual nature of their physical relationship (though it was very clear to the audience).
It sounds like the tour has lost a bit of the nuance.
