It is always best to get something signed giving you permission and a must have if it involves minors. I even got a release for the Amish kids pic before I would put it up. I know our local newspaper won't put in minors photos without signed permission and a phone number, news story or not if the person is recognizable. Sending a link to a simple release.
http://www.dpcorner.com/all_about/releases2.shtml#Permission You can always add a parent/guardian line. I would also put my company name, email and website address on it. Give them a copy of it and a business card. You can always run off a few of those before you go if you don't have any. Besides they might buy a print if they see it. Just run off a few copies and keep them and a pen in your camera bag all the time.
Now if it is a group of people in a public place I wouldn't worry as much. Though again a business card is always nice to do and cheap advertising.:evilgrin: