Well being a boy scout leader I can tell you I take a lot of action pics of the kids at camps and activities and the one thing I can suggest is TAKE LOTS OF PICS! Expect 2/3 of them will be trashed.
The little devils move when you expect it least, they move fast and they don't really care about your photographic efforts
Another thing I love to do, I'll usually find the most outgoing kid and lend him one of my point and shoot cams (Optio S6) and tell him to go nuts with it. That will always bring in some pretty unpected good pics to add to the album or slideshow I give to the parents.
Pat