A popular slumber party game for 12-year-old girls is Truth or Dare. The rules are easy: players ask each other "Truth or Dare?" If the player opts for "truth", she has to answer a question truthfully. And if she chooses "dare," she must complete a dare posed by other players.
An interesting game to play at a slumber party is a simple "Treasure Hunt". All one needs is a list of clues that the girls follow to move from one room to another. The treasure can be a pizza or some other treat. "Pass the Orange" is another exciting game to play, where the girls are divided into two teams and are supposed to hold their hands behind their backs. Next, one of the parents places an orange under one player's chin. She has to then pass the orange to the player next to her without using her hands.
Since most girls like makeup, its easy to get them to play the "Blind Makeover Game", where a blindfolded guest performs a makeover on another girl. Make sure there are enough makeup removal wipes available. Finally, a game that every 12-year-old girl loves to play is the telephone game. Make the girls stand in line, and have the first girl whisper a phrase to the next girl in the line. The phrase gets passed on until it reaches the last girl, who announces it to everyone.