Well, this lens absolutely has some issues (which, in my experience, is unusual for Sigma lenses; I'm a big fan).
At 1/1600 of a second? 1/2000 of a second? Your images should be
ridiculously sharp. Minor motion on your part, or on the part of the kids you were shooting, would be a non-factor. 1/1600 of a second is unbelievably fast. I don't even shoot that fast when shooting auto racing, and I get some really sharp images. Here's a photo I took at Daytona in January. Pretty sure my ISO was at about 400, and shutter speed was probably in the 1/250 range. The important thing to keep in mind is that I'm panning along with a car that's travelling around 160 mph:
Could that be sharper? Yeah, probably. But it's still pretty good considering both my camera and the car were moving.
The basic rule of thumb is this: If you're shooting with your lens zoomed to 200mm, you want your shutter speed to be a
minimum of 1/200 if you're using a full-frame body (I shoot with a 5D). If you're shooting on a 1.6X crop body, which I believe your camera is, you want a shutter speed of at least 1/320 (200 x 1.6). You're
far exceeding that, and your images are pretty soft all around. I
have to believe it's an issue with the lens and/or camera body, because I just don't think you can move fast enough to cause motion blur at 1/2000 and, let's be honest, that kid with the baritone sax probably ain't all too nimble with that thing around his neck.
The only other thing I can think of is that you're shooting wide open at f/2.8. It wouldn't be unheard of to get soft images if you're shooting wide open, and it can be an issue even with Canon's L-Series lenses. Try stopping down to f/5.6 and going to Av mode. If your ISO is high, you should still get a more than fast enough shutter speed.
I'd be real interested in seeing those results...