UV filters really don't do much for digital... and unless you actually spend good money on one it might even degrade the quality of your pictures by giving them less contrast and less sharpness. the only benefit to a UV filter is it protects your front element.
I too photograph youth sports (mostly football) mid day, see my site:
I shoot with my 40D and 70-200mm L f/2.8 IS, usually i shoot on AI servo and i only use the center focus point. In fact no mater what kind of shooting i do i use only the center point. But i usually shoot Av mode at f/2.8 just to knock out the back ground which can be distracting in some cases. I usually shoot ISO 100 to keep noise down unless i need faster shutter speeds.
You should post some pics, I'd be happy to give you my opinion on them and I'm sure other members would as well.