Soccer is a field sport, and as such, action can be very far away, especially if tou are in the stands and the stands are at a common high school three-use field with a track, a football field, and a soccer pitch, all combined...the stands can add 40 yards distance to the action.
One approach is to gain sideline access, but then again, that's not a complete answer. Soccer is one of the fastest-moving sports. The ball can be kicked 50-60 yards and moves with amazing speed on long passes, so one second the action is right in front of you,and the next, wayyyyyy down at the other end.
A lot of people like a long zoom lens, like a Canon 100-400, Nikon 80-400, Sigma 100-300, Tamron 200-500, or Sigma 50-500 or Sigma 150-500 for covering daytime soccer, from a distance. One option is to shoot from afar, and crop-in at the computer to get a closer, better-looking image. These lenses are not cheap. And they are not suitable for low-light, artificial light soccer.
You might have to resign yourself to shooting only the closer action, when your kids are reasonably near to the camera, and going with one of the more-affordable lenses like a 70-300mm f/4~5.6 variable maximum aperture zoom lens in the $199 to $500 price range. The Nikon D90 and the EOS 50D you have mentioned are fairly close competitors with similar AF systems. In terms of an affordable,good lens for daytime soccer, there are all the choices listed above, plus a few others.
If you want to get into low-light sports, affordably, the lowest cost is to buy a used prime lens of good quality, and either get close, or shoot from a bit father back,and crop. I am speaking of 85mm 1.8 lenses, something like that, a medium length, high-speed (ie wide f/stop like f/1.8 that allows a lot of light to enter). There just are not that many lenses suitable for low-light (ie nightime,stadium lights) soccer in the under $1,000 price range. IF the matches are in the daytime, you can shoot a lot of rigs and do good work...but at night, it becomes more difficult to get "good" shots. But then again, how good do they have to be??? I think a lot of experienced shooters tend to over-estimate how "good" a shot of one's own kid needs to be to get enjoyment out of the photo, you know??? If you can get close, pick your moments, and not expect to be able to cover the entire soccer pitch form point A to point B from one camera position, this whole issue becomes a lot easier for you; with patience and from close range, you could get good shots with a d-slr and a 50mm 1.8 lens. Maybe only three or fur per game, but they'd be clear and crisp, day or night, on close -range action.
