If I would be breaking up a family then probably, also if it was a mother and her baby, but a women / child, probably not. They don't have any intrinsic greater right to life than I do, and we don't have time to weigh up who did more good / less bad.
I don't see how a child would be alone though, so if a child was trying to get on they'd have likely followed a relative, and this would become immediately apparent when someone starts being hysterical next to me.