Here's what I can summarize regarding the issue:
1) Is it legal to take pictures of a kids at the local park or school's soccer fields without first seeking permission? The answer is a clear "yes". It's not considered an "invasion of privacy" because you have no reasonable expectation of "privacy" in a local park or similar area. Minor or adult, there's no legal distinction. It's not illegal for me to take a picture of your kids playing in a park without first seeking your permission (written or otherwise). (*)
2) Is it "legal" to sell such images on the web without permission. I'd say the answer is a qualified "yes". The qualification is that if a person (or parent of a minor) prominently featured and easily identifiable in any of the images objects, you would probably be best advised to remove their images. If you don't comply, you
could be subjected to being sued. (People like Derek Jeter aren't the only ones afforded legal protection from people selling photos of them without authorization.) But this would be a civil matter instead of a criminal one. Would a sports photographer "win" in this case? Perhaps not... but there would no doubt be a lot of factors to be considered by the jury before deciding. Such factors could be the prominence of the subject in the photo, the use of the image (selling on the web vs. selling the image to be used in an ad campaign by a sporting goods chain), the amount of revenue generated by sales of the photo, etc. The bottom line here is that the lack of a release only becomes an issue when someone objects to the commercial use.
A smart shooter would post the images en mass and then remove ones from people that object. Smart shooters, as mentioned, also try to reduce parental fears by password protecting the galleries so as to reduce fears that "perverts" are downloading pictures of their kids or people who would prefer that certain people not know where they are living don't want their location "blown".
3) As others have mentioned, there are times were permission to photograph may be included on the player or tournament sign-up form. However, all this really does is prevent any later objection or claim of compensation of the subject(s) for commercial use, it doesn't make the actual taking of the photos "legal".
* = One exception, is if the sporting body has a contractual agreement with a photographer that gives them exclusive rights to sell images from the event or exclusive access to shooting locations. As mentioned previously, in the US "public" lands can be considered "private" in terms of determining trespass. If a local soccer league has leased the fields from the parks department for the summer, they will most likely have control over who can stay and who can be ordered out... and the police will back that up. In such cases, a freelancer as described by the OP could find themselves bounced from the playing fields. The same true of school grounds. The Principal and senior staff (AD, VP, etc.) have the legal power to ask anyone to leave the grounds at will.
However, it's important to note that this doesn't make any photos actually taken by the freelancer before being booted "illegal", it just means that the organization may act to prevent the shooter from remaining on the grounds and shooting. If I manage to take photos from the games and post them on my web site, the soccer organization cannot force me to remove my images simply based on their contract with an "official" photographer.
High school athletes are mostly under 18, have you ever heard of Maxpreps?
http://www.maxpreps.com/FanPages/NationalGalleries.mxp
BTW, MaxPreps is owned by CBS.