Wow, this argument has gotten fairly intense.
I think it's rather likely that Vin and Elend have had or are going to have a child, after all, there's no form of birth control that we know of, and it would certainly explain Vin's absense. It also lead me to believe she's pregnant (if not she could leave the child with someone she trusts), or is just uber paranoid about leaving it, that could also cause Elend to toughen up a bit.
As for what could a three month old child do, how old was Jesus when he began to change the world
So ya, I think a child is possible, but the only thing that makes me think no, is it doesn't seem like something Brandon would do, but who knows, he did just have a baby of his own, although he completed this story before that so who knows.
EDIT: And I don't think that the child is the hero, that just doesn't seem likely to me, especially after chapter 1's bump. And now I'm thinking maybe there is a child, or one on the way, it seems like some of the twist brandon would just love, and think of the character development, Vin goes from untrusting abandoned child to soon-to-be mother, the tensions would be crazy!