one resource i suggest getting in contact with is Bert Krages (
http://www.krages.com/). He's a pretty well known art and photography attorney.
this topic has been discussed on sports shooter quite a few times. evidently, in the states the general rule is if it's public ground, then it's ok - minor or otherwise. school fields are generally not public property - as are some club fields. it's generally not the act of taking the photographs which are of concern - it's what is done with the photos after. most people have their photo taken in some shape or another several times per day - generally without knowledge.
i often shoot youth sport for a few of the local papers. here in oz (and i suspect in most countries) a model release is not needed for most publications - newspaper, magazine, etc... again, usage definitely comes into play. obviously where minors are concerned, it's a whole lot more complicated. when shooting youth sport - we always shoot openly and publicly. if ever asked, i'll gladly show images taken. we are never confrontational, and in the event we are ever questioned, we request police be called to intervene. the *only* time i have ever been asked to stop shooting was when another photographer was claiming 'territory' and complained to the event operator. after a short discussion with the operator, i merely walked to the other side of the field and continued shooting. there are some clubs, in order to protect themselves, are starting to require signed permission slips.
edit: oh yeah - we also closely follow the Standing Committee of Attorney General (SCAG) commision for children and young people guidelines. the following is a quote taken directly from their guidelines:
The photographer or camera-person owns the images they take (under the Copyright Act 1968
(Commonwealth) s35(5)6). If a photographer takes a photo of a child or young person at an event or in
public and the image cannot be classified as indecent, they hold the copyright on that image and can
publish it without asking for consent. The Internet is not governed by the same regulations as print or
broadcast media, so photographers can display photographs or vision online.
not directly related, but i have been stopped from shooting by the Anti-terrorist patrol, by Harbour Bridge security, and a security guard at Luna Park (claiming my Bronica is a professional camera so it's illegal).