N.B.
I have a friend that has 10 kids, when they do a family group photo she and her husband wear a solid color and sit/stand in the middle.
Then each of their children's families wear one color. Her oldest's wife and kids would all wear black pants/skirts and red tops. The next one would wear black pants/skirts and dark green tops. Then so forth. Contrasting colors next to each other so they don't blend into each other. The kids don't need to stand in any certain order for this but if they do a line up for a sibling photo you might have them be color coordinated like a rainbow or something, just not stand next to each other in the large group photo.