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Continuity mistake: In the scene where the women are putting the eggs into the car and Ouiser walks over to Annelle and begins to converse with her, she is holding a carton of the eggs. The two front corners of the eggs keep changing colors as the shots keep changing from Ouiser and Annelle speaking.
Answer: I believe it is because the child playing Jack Jr. was basically crying through this whole scene, as you can see. It's most likely his (actually her, as the real baby was a girl) real mother holding him to calm him down. Also, when the family is saying "Blow, blow," you can hear a female voice that is neither Julia Roberts or Sally Field.
When she is sitting with Jack Jr, it is before she goes to get her hair cut. When she tells M'Lynn she wants to cut her hair, she is wearing it in a banana clip and you can see the hair past her shoulders. When "she's" sitting with Jack Jr, there's no hair to be seen. Total stand in.
Daniel Camp is the actor who played Jack Jr.
Along with Daniel two other children played Jack Jr, C. Houser and Clara Gabrielle.