Nina isn't good with kids.
It isn't that she doesn't like them; she's just uncomfortable around them. She doesn't think much about it until Dr. Rosen starts asking about her own childhood.
"You faced a lot of adversity in your youth," he says, "and that will always be part of you. Do you think you could find a way to be kind to the little girl you were then, without harming the woman you are now?"
That's when she realizes that when she looks at small children, she sees herself as she could have been, if the world were just.