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Again...not a great diagnostic image... The girl in #7 really does not seem that sharp, to be blunt, especially if you look at the grass by her feet.
However, if your son is the one at the back then it's really asking a lot of any camera to pick up on him if you're using the expanded focus points illustrated in your images above. If you had single point focus specifically on him, perhaps, but you have the ref AND #7 getting in the way if you're on your 'expanded' focus points, as you seem to be. Even with my Mark IV, with the considered size of him in the frame and those others moving in front of him, perhaps, this would be a tough focus with her arm in front...and the white of his jersey in front of a white wall.
What exactly do you have your focus settings on? Like I said, above you really seem to have a wide range of focal points for your camera to choose from, especially shooting sports.
Also, I noticed you're on Mark III Firmware 1.2.3 The most recent version, I believe, is 1.3.0 You can access it here: http://canoncanada.custhelp.com/app/answers/detail/a_id/7067/~/eos-1d-mark-iii-firmware-update