I've shot thousands of shots of kids soccer. Until my kids got to U12, I was way ok on a crop camera with a 70-200 f/4L. This lens is terrific, it is light weight and it will do great.
You don't really ever need to reach ALL the way across the field from corner to corner - those shots are rarely that good anyhow. Spends some time down near each goal line to get what you want. I find that when I'm shooting, I'm basically shooting in a third of the field anyhow. Rarely do I find a perfect, clear shot all the way down the field. There usually are other players in the way or just because it is a hazy summer day, it isn't that great anyhow.
I now shoot with a 300 f/4L IS but I do that on a 5D so it is essentially the same thing as the top end of the 200 and it works just fine. I *may* go for a 400 f/5.6 next year, but I doubt it.
Do invest in a decent monopod to make this all work for you. Even at fast shutter speeds for soccer (1/1000s or better) you will get sharper shots with a monopod. Also, if you are shooting a lot, you will not get as tired.
If you want to go for the 70-200 f/2.8 then if you invest in a TC, you will essentially have a 150-500 at very,very little loss in IQ or AF speed. If you use the 1.4 on the f/4 version same thing, but you will be at f/5.6 instead.
At any rate, I think the 70-200 is a GREAT lens for kids soccer. No worries on this and I'd recommend it with no hesitation whatsoever for a 1.6x camera.
Give some thought to shooting primes, however. The sharpness is great and there really isn't all that much of an advantage to using a zoom once you think about it.
J.