A few years ago I was commissioned to paint the portraits of two sisters. Children won´t hold still for you, so I worked from a family snapshot for one and a studio photograph for the other. I knew the kids quite well, as they lived across the street from me. Otherwise I would have had to ask for a whole bunch of photographs to get to know them better - I do the same when I have to paint a portrait of an adult whom I don´t know well and who cannot sit for me.
And being familiar with their personality was very helpful.
The older sister, the blond girl, was very outgoing and full of spunk, her little sister rather sweet and shy. Their mother left the choice of backgrounds up to me and was quite happy with the prickly cacti in back the older and the soft meadow behind the younger child.