I'd also be careful about selling them to anyone but the parents, and then only for personal use. If you sell them to somebody for commercial use, the kids could have a claim against you -- unless they (or their parents acting for them) sign a model release. But if it's for artistic or personal use, taken from a public place, there's no problem.
There are a number of causes of the growing problems of taking photographs in public -- the perception that there's a pederast under every bush, taking photos for salacious purposes; the perception that terrorists couldn't blow up a building unless they first had a picture of it, even when, say, postcards of the same view are sold to anyone who wants one; the fact that we live in a celebrity culture, and there are constant stories about celebrities trying to control access to, and use of, their images, and if celebrities can do it, why not Joe Blow?; a weird sense that photographs may violate privacy in a way that simple looking does does not -- a couple may be seriously necking and petting in public, with a parade of adults, teenagers and children going by, goggling in amazement or disgust or deep personal intersst, but if somebody takes a picture, their privacy has been violated.
Also, if there's money being made, there's a general sense that everybody should get a cut. That's what's causing your problem. Who gave the people who signed the "contract" with the "official photographer" the rights to any images? Thge parents? Not if they're happy to buy pictures from you. So they're selling something that they don't own. If, however, they rent a playing field, they may restrict your access to the non-public parts of it (that's how the NFL keeps every amateur photographer with an SLR out of the end zone, even in a public stadium.)
JC
