Evidently this used to be a common studio technique:
http://swapatorium.blogspot.com/2007/04/lazy-boy.html

It was certainly a oft-used technique in my studio in the late 70s. I did lots of kids under 6 months old who were incapable of sitting on their own. I'd put Mom behind my posing table, have her duck her head low and cover her arms/hands with a blanket. Then she supported the child. Worked a treat! And usually kept the child at ease too, since Mom was so close "at hand", so to speak. Sorry I don't have any samples...