Are you sure that something else isn't set wrong? Maybe they are being overexposed due to exposure settings. If not...
70-300? Can I assume you're taking photos from a distance?
For shooting sports outside, where sun can change quickly or exposure can change depending on which direction you're looking, I'd suggest using the AV setting. This bases the shutter speed on the f-stop setting that you choose. For sports, most people like a shallow depth of field, so set it as low as it will go. Some 70-300s will be able to go to 2.8, many others are in the 4.5 range.
Setting it low will have your subject in focus and the background blurry. Once set in AV, the camera will automatically choose the shutter speed that it believes will expose your shot correctly, based on how it's reading the lighting.
The other consideration will be shutter speed as a running (moving) subject will require a faster speed. If they're bigger/older kids, it's likely that 1/320 will be the minimum speed that you can get away with. The camera will take care of this for you, but if it's an overcast day, the camera may have to choose a shutter speed around, say 1/100, which will most likely give you blurry subjects.
If you're seeing slow shutter speeds due to lower lighting, bump up the iso setting to 200, 400 or 800 until you see the speeds in the range where you need them.
Hope this helps. I've shot thousands and thousands (and THOUSANDS) of lacrosse, baseball & hockey photos.
I love talking camera geek stuff.