If she's a responsible kid, let her go... with some parameters. I've been to the parade before and it's alot of fun. In general, it's about the crazy costumes and people-watching.
Make sure that she has a cell phone that must remain ON at all times. Have her check-in with you at certain points and make sure that she has enough cash to grab a cab and head to train quickly if something goes wrong. You could also check-out the train schedule and decide which two train options are acceptable, that way you know what time she will be home and can somewhat determine the length of time she's there.
If you're really worried about it, head in to the City with them and find something else to do while they're there. You and your husband could catch a show or do some shopping and then meet-up with them to take them home.
Either way, if she was responsible enough to go before, I would say she's responsible enough to go again and recognize when a situation isn't safe. Remind her that she can ALWAYS call you to come get her, even if she's the drunk prankster... punishments for bad decisions can be discussed the next day... her safety is the priority.