It isn't common but the sensor can briefly fog in an extreme temperature change; if you get condensing moisture on other items it can happen but it is usually brief. That could have triggered the clean sensor warning.
1/4000 is far too fast of a shutter speed for normal sports usage and is probably the fastest speed for that body. In the daytime I typically shoot soccer and football (high school level) at 1/1250 or 1/1600 depending upon the light level while at night I drop down to 1/1000 and that is using a F2.8 lens wide open and it still pushes the proper ISO well beyond the range of your Rebel XS. For younger age levels of soccer 1/500 is very usable but faster is desirable when lighting conditions support it.
Getting good night photos is going to require you to use the minimum shutter speed possible (you should be able to get some images at 1/500 and maybe 1/250 at the wider end of your lens) that will provide reasonable sharpness. You need to get as close to the action as possible to maximize the level of light reflected from the players to your sensor. Illumination level follows the inverse square law so every time you double the distance from your subject the light level is divided by four. Of course the same applies to the distance of your subject from the light source so optimal for nighttime sports is to find a location where the subject is close to a light source and you are close to the subject. You also want to fill as much of the sensor as possible with the desired image to avoid cropping an image that is already noisy and lacking in detail. I believe the maximum aperture (F stop) of your lens decreases as you increase the magnification so if you can keep it at the F4 end of the range (wide rather than telephoto) that will be of major benefit.
It is very difficult to get action shots in dark light. Your focus system will struggle and the images will have increased noise and decreased color. Today I am shooting a high school soccer game which starts at 5:00 and for most of that there will be decent light and it will be a fairly relaxing event. Then I am going with the football team to shoot a game on a field that is new to me and I will have the camera set up with Av fixed at 2.8, shutter at 1/1000, and the ISO will adjust as needed and if I am lucky most of the shots will be at ISO 12,800 or below but I am not counting on it.
One trick you can use for night events is to figure out which part of the field has the best lighting and stake out a position near it and wait for the action to come into the lights. My daughter was in her last year of middle school last year and was invited to join the high school varsity soccer team for their indoor club season and I tried to shoot it with my Canon 1D Mark II that had served me well for years but with a maximum usable ISO of 1,600 (3,200 looks terrible with that body) the results were basically worthless and I moved up to a new 1DX Mark II. The only good shots I got from the indoor games I shot with the older body were in one well lit portion of the playing field.
Good luck in getting some great shots of your grandkids!
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
