Photo Lady mentioned taking photos at fairs, festivals, etc. Try looking up the event and there should be info. somewhere (website, media guide, back of tickets not so much anymore!) about taking photos, etc. I would think though if someone asked you not to take their picture, especially of kids, it would be better to wait til they move on, then take the picture of the booth, display, etc. Or find something else to photograph and come back in a few minutes.
I've done sports/events and usually there's a disclaimer available somewhere that there may be TV cameras there, photos are allowed, etc. so people know they could be seen at the event on the local news or in the local newspaper, or their pictures may be taken or used by the team/venue that could show them at the game/event.
Photographing events usually involves timing. I found that heads would swivel, and kids (and people who had been to the beer booth) would wave and want their pictures taken, then they'd realize I wasn't all that interesting and go back to watching the game. Then I could get some pictures. So it takes some watching and waiting.
In general it depends too on usage. You may be able to take photos for personal use, which could include usage as a print (for the buyer's personal use). Anything for commercial (business, advertising) use or for retail use (mugs, T shirts), releases would be needed/permission given.
Try
American Society of Media Photographers - Homepage or PPA for info. on usage, releases, etc.