Is she there so that that while Harold and Marcellus are talking about Harold's preference for faster women like Hester Prynne, having a child in the scene makes it then more G-rated? Are they being more genteel than they might be without her there, so Willson could get in a slightly risque song? Is Amaryllis usually on stage in that scene?
Of course, now I'm envisioning a production of "The Music Man" as seen through the eyes of slightly worldly-wise Amaryllis, who understands what's being sung about in "The Sadder But Wiser Girl", and her imagination of what's transpiring between Harold and Marian when she's not on stage. "Goodnight, My Someone" indeed!