I can clearly remember playing this game when I was about 9 or 10 years old. It was my favorite game to play at school (in the classroom that is).
Ideally this works best if you have kids sitting down at desks. Depending on the size of your group, pick 3-4 people to stand at the front of the room. Everyone else then puts their "heads down and thumbs up". By "heads down", we also mean eyes closed so they cannot see what is happening in the room. By "thumbs up", we mean having their thumb pointing up so it can be grabbed.
The 3 or 4 people up the front sneak up and squeeze the thumb of someone in the room, and then sneak back to the front of the room.
Once that's done, everyone opens their eyes and the people who's thumbs were squeezed have to pick which one of the 3/4 people it was that squeezed their thumbs. If they guess right, they get to go up the front and the person who was 'guessed' sits down.
Repeat as many times as necessary (or until it gets old)
It's a pretty simple game and thus works best for younger people - as I said, I loved it as a kid :)