I'm pretty sure that the children available for adoption probably won't be pre-existing NPCs, I'd bet they're actually customizable (name, gender, etc).
But for this thread's sake (if any NPC child was adoptable, whether they have family or not), the first kid I'd adopt would be Haming. If you don't know who he is, he's the boy from the opening scene in Helgen. His father Torolf died during the attack so now Haming's living with his grandfather Froki Whetted-Blade.
My second would be Hrefna, the girl in Darkwater Crossing. She's really opinionated which I like, she doesn't like her mother's support of Ulfric.
These are two of the few children in game that I like, so that's how I based my decision.