If you do indoor sports, you will need a fast lens. Nikon or Canon? It's really up to your preference. There is not going to be much noticeable performance difference, both can handle these shots quite well.
I have the Canon 30D and usually use the EF 70-200mm f/2.8L IS for indoor and outdoor sports photos for my 4 year old. The lens is much more important for low light action photos than the camera body. You need at least f/2.8 or faster for indoor action shots. Your 70-300mm will not be adequate for indoor action shots. If this is too expensive, then there is the EF 100mm f/2.8, and you can use the 70-300mm for outdoor sports photo. The 18-55mm IS kit lens is usually okay for general purpose use, but will need flash indoor. I have the EF-S 17-55mm f/2.8 IS which is a great lens for general and low light use. The EF 24-105mm f/4L IS is a better match for the full frame D-SLR. For sports photos, you will typically like to have at least 5 fps but you should do okay with less.
So Canon 40D/50D or Nikon D90 are all good choices. If you get the fast prime tele lens and use the old Canon tele for outdoor, then you can save some money buying the Canon system. But if you are going to get all new lenses and camera bodies, then either one will be good.
I have had a Canon point and shoot for a few years and am getting ready to buy a DSLR. I still have my 35mm EOS Rebel with 35-85mm and 75-300 mm EF lenses and filters and a teleconverter, I have really missed having an SLR but have been waiting for the prices to drop. My dilema now is what to get, I would say I am definitely more than a newbie but not quite a pro. I have taken a couple of classes and enjoy playing with all of the settings. Should I lean towards another Canon since I have a lens that I can use already. I know the 35-85mm EF wont do me much good with the focal length factor and the lens that will come with it. Is having the 75-300mm worth sticking with Canon? My children do gymnastics indoors and I would like to be able to shoot that, no flash is allowed though. I also do some shots of baseball and other activities along with the usual family events. I am usually designated as the family photographer. Can I go with the Rebel XSI or should I go to the 40D? Is the extra $300-400 worth the additional ISO stops and buffer times? I see most people rate the Nikon D90 higher, should I just scrap the Canon and go with Nikon. I have been chewing on all the data for a while and have been reluctant to pull the trigger.