@KVbarkley
Concentrate on the shots you can get rather than those you would like to get. The combination of lens aperture and camera ISO performance available to you is going to make capturing high quality action shots under typical high school night field lighting pretty much impossible. On a better than average illuminated high school field, I will shoot at ISO 2000 with a F2.8 lens wide open and shutter speed set at 1/640 which is fast enough to freeze most action but on a lot of fields I am up to ISO 6400 with this combination. I shoot night high school football in full manual mode with the lens wide open and shutter speed at 1/640; if the field is fairly evenly lit I will also lock ISO and adjust when I process the RAW files. For fields with highly uneven lighting I let ISO float. I lock shutter speed because I have yet to take a blurred photo that is worth keeping 🙂
I am not familiar with all of your lens options but for your primary choice, I would take the one that will go as far into the telephoto range as possible with an aperture of F4 and rely primarily upon it. A shutter speed of 1/500 is very marginal for high school football action and anything below is going to have a lot of blur but with your ISO limitations I would set the ISO as high as possible and go to Av mode realizing that you are going to lose a lot of photos to extreme motion blur. Even 1/640 doesn't freeze everything but when you can go that fast within lens and ISO limitations it is very usable.
Concentrate on the shots that you can get rather than what you would like. Capture the pre-game warmup where there may still be some natural light and the action is a bit slower. Capture player reactions after a score or turnover or even the cornerback checking off with the line judge because these are situations where a slower shutter speed will suffice. Concentrate on staying dead steady because with the problem of motion blur there is no margin for adding any operator induced blur.
Ernie gave you excellent advice that location is key and that is even more critical with gear limitations. You want the desired image to fill as much of the viewfinder as possible because cropping and high ISO noise/loss of detail do not play well together. But even after Santa brings you better equipment, location is still critical to get the most of your new gear. Like the players, you have to fully concentrate during the game to improve your odds of reading the play and being where you need to be. Last Saturday I enjoyed shooting an afternoon game for a friend whose grandson played what turned out to be his final high school game. It was nice to shoot in decent lighting conditions for once but one of the best shots of the day was when I was sure the next play was going to be a pass and I ended up being in the perfect position to get a good capture of an interception. The other photo is an example of the type you could get where a slower shutter speed wouldn't ruin the image. My friend's grandson stepped in as a holder because of high wind conditions so even if lighting conditions would have required a slower shutter speed, he was virtually frozen and would have still been sharp even though the kicker would have been blurred. This is a type of action shot you can capture with gear limitations forcing a slow shutter speed where you would have motion blur but not of the most important element in the image.
As you get better equipment, you will run into the limitations of that gear also because there is no absolute perfect. The knowledge and experience you gain in working with your current setup will pay dividends as you continue to do photography and move up to more sophisticated and capable gear. Hopefully as your child advances, there will be a nice daytime playoff game 🙂
Rodger
"I would take the one that will go as far into the telephoto range as possible with an aperture of F4 and rely primarily upon it."
She has nothing that has f4 and tele.
"I will shoot at ISO 2000..."
Her camera tops out at ISO 1600 so 800 is probably the best it can do and get reasonably acceptable results. It is also just an 8MP sensor. She's up against it for sure.
"Last Saturday I enjoyed shooting an afternoon game ... It was nice to shoot in decent lighting conditions for once ..."
I am jealous! 
Never, never done that.
"Concentrate on the shots you can get rather than those you would like to get."
I wish I had said that because it is so right. I used to shoot and XT and XTi for years. They are capable but severely limited by today standards. This can be done, I did it. This is what falls under "know your sport" because you know what and where you have to be to get those shots. If you are just guessing or are confined to the bleachers, it's going to be tough.
