If you want to make money at spec shooting of kid sports, you have to have a system for on-site printing.
Check this out:
SportsShooter.com - Classified Item
Someone with a setup like that is going to be hard to compete with. They can sell their pics on location, and not have to hope that the parents will check out the website of some creeper taking pics of their precious little snowflake. You may be cheaper than them, but they are able to take advantage of impulse buying.
And again, cameras and lenses are cheap and prints are cheaper, so you are going to have a really tough time convincing parents to buy your prints for 100s of dollars when they could buy a 20D and a 70-300 for under $500 and take a decent pic themselves.