I say stick to your guns!! This isn't about what's going to happen the next time she wants to do something..it has to do with this time!! Kids aren't very good at judging their own ability to resist temptation, so why put them in a situation where they are tempted? And even though she may throw a fit now, she may actually be looking for an excuse to not go! Peer pressure (even if the others are not there to hear your NO) dictates she have a hissy fit, she'll silently enjoy her relief in making you the heavy and saving face while not having to attend something that would make her uneasy :D