Thank you, Suzie and Longwatcher, for your responses. How all this started was that lately I've been asked by some parents of my boys' classmates to shoot similar photos I did with my boys. They really like the posters of my boys on my wall. So when I show up at their event to shoot, some other parents think I'm a pro with that long white lens, and they ask whether I'd be interested in shooting their kids, etc. After providing these parents with a CD and sometimes poster prints, etc. and feeling really guilty for charging them (even for just some gas money!), I thought to myself that it's about time to get a SmugMug's pro account and let them do all the transactions, printing, etc. while I just concentrate on shooting. I love shooting, but at the same time I don't want to provide a free service all the time.
And if I happen to be at the location, why not shoot the images of all other kids, and therefore my thread topic question....
I guess I can either get the permission of an authority if I want to shoot the entire kids, or just limit myself to shoot only the kid(s) whose parents asked me to shoot.