The "big trick" to sports photography has always been learning the sport, and that's only a little less crucial now with, say, a 1DX or a D4/D4S than it was back in my manual-focus days. However, that's not going to be of much help when the kids you're photographing don't know the sport. Expect a comparatively large number of failed shots, and learn to laugh. A 400/2.8 L isn't going to help a whole lot until there's some statistically-valid chance of anticipating the action. (And it's godawful heavy and awkward as well as really good used car expensive.) As your photography develops and your athlete develops, you'll know when (or if) it's time to go for the big guns. In the meantime, a fast 70-200 (with a good teleconverter for some shot types) will fill the bill, and unless you're printing huge, don't be afraid to crank the ISO a bit to keep the shutter speed down. (Look at the pictures, not the pixels. There ought to be some sort of license required to zoom in to 100%.)