Originally posted by Peter Zack Yes post a picture to help determine this. I suspect the kids were too deep in shadow and being mostly black there was no contrast for the system to focus on. It happens and there's nothing wrong with the lens I bet.
^^^^^^
What he said.. Shadows on shadows in the dark aren't very autofocus friendly. I'll bet if you had aimed at a location where there was light vs dark in this situation, the lens would have locked right on. Besides that, it isn't really the lens that drives AF, but the camera's AF system. The lens is just focusing the light. Now, if you cannot AF in ANY situation, try a different lens.
In a situation like your photo (edit), I would have the camera set for Center Spot focusing (my cameras live on that setting), and aim for the area where the little girl's sweater is against her brother's shirt. Unless they are standing in total darkness, that should get you there every time.
Cute kids btw