This stuff gets really REALLY tricky, really REALLY quickly. It can get very rough on the amateur photographer's end. Best advice I can give you is to approach the coaches beforehand, introduce yourself, tell them what you'd like to do, and maybe even offer to send a photo or two of the kids in action back to them once you've cleaned everything up.
They may say that it's just not a good idea, and unfortunately, you'll have to call it a day. But, if the coaches like the idea and give you permission, the resulting disagreement you went through above wouldn't have happened. All you would have had to do is let the woman know that her child's coach agreed to let you take some photos in return for a few finished photos via e-mail for free.
Past that, you're no longer involved at all. The disagreement/argument/fight is now between the parent and the coach not between you and the parent. In other words, while the coach may still ask you to leave after the parental complaint, you do have a "get out of jail free" card to play as soon as she says the first word.