My DSIL's mother did this. My DD and DSIL would have tolerated watching the favored siblings open their photo thing and charging docks while they opened an expired collection of jelly and a beef stick, but apparently there is a process of gift giving in DSIL family. Everyone gets a present, opens it individually, and it continues until all the gifts are opened. They endured watching their lovely little girl open the one toy from the dollar store while the favored grandchild opened MANY gifts. My DSIL have never returned for a CHristmas.
I have no idea why anyone would do this to a child, but to make sure that your son knows he is not the favored child in such a glaring way and hurting his own precious child is not the way to foster a relationship.
My beautiful DGD is 15 year old now, and she does not remember how hurt she was while she quietly and politely sat in that circle with one little pony thing, but she does know that she does not have the same relationship with her grandmother that she has with her Nana and Pa. It is impossible to undo senseless cruelty, and my DD and DSIL have ensured their baby girl was never hurt again.