
Continuity mistake: Right before Ben jumps off the cliff, he drinks a small cup of coffee and places it back on a saucer that Jonathan is holding. When the shot cuts, the saucer is still visible in Jonathan's hand, but the cup cannot be seen on it. The cup then reappears in the next shot.
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.