If they're anything like the socks around our house, I'm sure those could suck the life out of just about anybody!
That's a marvelous party idea, Jughead! Sounds like so much fun. Have you guys found the recipes for pumpkin juice/butterbeer online?
The US Potter conventions (I was part of the administrative staff for one once, before I quit. Don't ask.) have an official ground quidditch game and I'd bet they'd be willing to share the rules with you.
We've been to many a Potter book release at bookstores... and we've seen some fun activities. Build your own wand, with string/pipe cleaners/etc. Potions classes... kids could make slime, or do those simple chemistry tricks (color-changing, dry ice, etc.)
I'm sorry I'm not nearer, as to portray the Fat Lady!
And about the whomping willow stuff.. two seconds of glee per kid ain't bad, as long as you have somebody willing to do it. We did a live scarecrow once, and the bursts of terror/laughter made all of the waiting in-between completely worth it.