That's Pollyanna's dress!
Just noticed after rewatching the movie via Amazon streaming. In the church scene, Lallie Joy is wearing Haley Mill's dress from POLLYANNA. They just changed the sash and the piping on the collar. I've heard Walt Disney was cheap about costuming and made Lisa Whelchel (New Mickey Mouse Club) wear old, undersized shoes that hurt her feet in her "Won't Somebody Dance With Me?" number.