I think most flyers would put their seat up during meal service. There fact that the child behind was likely served early makes it difficult to know that was the case. Also that Mom was assisting the child.
But once you were clued in, you did the right thing. As everyone else around understood, Mom just went off and couldn't let it go.
You know modern day travel is probably the ultimate proof of the validity of Sartre's famous saying "Hell is other people."