Can you tell use what setting were used for that shot?
I believe your problem is that the Depth of Field (DOF) was just too shallow for the boy to be in focus.
DOF is the range of distance (from the camera) that will be in focus for any given photo. For example, it might be from 6 feet to 8 feet. Anything closer or farther away from the camera, will get progressively more blurry.
DOF is controlled primarily by the aperture of the lens. A larger aperture (smaller F number) will give you a shallow DOF. A smaller aperture (larger F number) will give you a deeper DOF. Also, the focal length of the lens will have an affect on DOF. The longer the lens, the more shallow the DOF can get. There are also factors like how far away the subjects (focus point) is from the camera....and the farther something is from the focus point, the blurrier it will be.
The DOF extends 2/3 of the way past the focus point and 1/3 back toward the camera...which is probably why the boy is slightly out of focus.
So in that same situation, shooting a group of people, you will want to use a smaller aperture to increase the DOF and get them all in focus. Try F8 or maybe F11.
Now, the problem that you might run into, is that when you choose a smaller aperture, you need a slower shutter speed (or higher ISO) to compensate (to keep the same exposure). If you shutter speed drops too low, you can get blur from moving subjects or the movement of the camera in your hands. So you need to balance your need for DOF with your need for fast shutter speeds. It can be hard with a lens like this.
It isn't a top quality lens but it's better than the 70-300mm lenses without IS. I don't think this is a problem with your particular lens...unless you specifically locked the focus on the boy.
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