In reply to Fredt:
you most likely won't have a lot of light and you might well be quite far away.
That probably means long focal length and slow shutter, which is a combination you're probably used to managing with wildlife and will likely mean that motion blur is the main thing that you'll want to be worried about, with DOF, high ISO noise and shooting at an aperture where your lens is sharp being quite low down the priority list.
I'd take both lenses if you can, work out what focal length/lens you need by framing while other people walk up. I'd shoot in aperture priority, wide open, with the highest ISO you can get away with, bearing in mind what size you'll be printing/sending (for email/social media or family albums and even 8x10 you can get away with a lot of noise as you're averaging pixels from a much bigger image). Take a lot of shots, especially if you can't get your shutter speed high enough to avoid blur, some will come out sharp. It might be worth briefing your daughter to walk slowly.
Don't worry about the lens being ostentatious - you'll be in a room with a few hundred people dressed as wizards! You'll be cursing if you need the big lens and don't have it.
Have fun and good luck!