The baby didn't look scared at all, they looked like the caption says. they got emotional from the mother singing. Maybe tears of true human emotions you know, since it is a human, maybe it was just crying out of FEELINGS.
That don't include being scared, it wasn't like the baby was screaming in fear, rather just shedding a few silent tears.., in what appears to anyone with a brain, out of sadness to the song....so anyway. seriously this is so cute and the mom has a great voice.