My classes average from between 50-70 children at a time. If I am doing a large group game that I could play for 15-20 minutes as a main game, it needs to be able to handle that amount of kiddos or easy ways for kids to get into the game and staying engaged.
Snowman USA (Teams is best for a longer time frame.)
Switch Bonk (If you let teams play against each other, and then rotate teams to play other people every few minutes.)
Three Passes (If you let teams play against each other, and then rotate teams to play other people every few minutes.)