Factual error: The opening states the location as Bristol, England. Santa is drinking in a pub on Shirely Street (neither the pub or the street exist in Bristol), then shortly after is shown leaving from the rooftop, on his sleigh, with multiple high rise buildings in the background. Bristol does not have high rise buildings in the main city centre, only a few blocks of flats, which all have flat roof tops, not like those depicted.
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.