Glass has got to be the biggest hazard for any kids' party. It's not only dangerous to break, but also tiny shards of broken glass are almost impossible to clean up before they work their way into little feet and hands. Vacuum cleaners won't help, and no matter how carefully you sweep, you can be sure there are going to be some tiny pieces you've missed.
Here's a tip to aid with glass cleanup. After sweeping up the biggest shards, dab the affected area with a slice of bread or two. The soft, doughy, texture of most sandwich breads make them soak up broken glass like a sponge -- better than a sponge, even. Just make sure no one eats the bread afterward.