Has she always been this way? Did you take her out to meet other dogs when she was small (socialize her)? Whatever you do, do not put pressure on her leash to keep her in one place when other dogs approach. Let them meet on their own terms. Sometimes if you tense up on the leash, the dog will think that it needs to defend you, and then you can have some aggression issues.

I think that if you can take her out lots of times to places that have other dogs around, she will gradually come around. Just let her take her own time to do it. I would take her to the Labor Day Party, and just be prepared for her to hide behind you most of the time. Puppies can intimidate older dogs, since they are sooooo confident that everyone loves them and wants to play. A 6-year old dog that's never been around puppies is going to be terrified of this behavior (what if it's going to attack me? , what do I do? ) Just be patient and try to make it as calm as possible (i.e. praise and treats when she tentatively sneaks out from behind you to sniff at the puppy or whatever other tiny progress she makes). If she has a crate, I would bring it along, so that if she starts getting really stressed out you can put her 'home' in the shade somewhere so that she can relax in relative privacy.

Good luck, and keep us posted