I like this image. The boy is in focus and as the central figure he is correctly placed for this effect. If this were a 'zoom blur' challenge, I would give this a high vote. However, since it is a 'motion blur' challenge, I will give it a lower vote.
I think I understand what you were trying to do here. It is like parent spidey sense on alert right? I think having the child almost directly in the line of your photo hurts it though. The first thing I see when I look at the photo is a zoom in on two people. Your idea doesn't strike until I read the title.