For 2–4 people
Dominion — Rules
For numerous people
Ligretto — Rules
Werwölfe — Rules
Kartenspiele and Brettspiele are a disaster if you don't understand any of the games yourself. Familiarize yourself with what's there before you offer it.
Along with getting to know the rules, figure out how many kids can play each game and how many games you can comfortably supervise, then figure out from there how many kids you can take. Otherwise you'll get 30 T's who just wind up stabbing each other with puzzle pieces.
Figure out the vocab for the games beforehand. There're few things more embarrassing than having a kid ask you how to say a vital piece in a game and you having to make something up so they can play.