Complex sentence: At the beginning of the game, the 12 kids were paired off into six teams of 2 (each composed of one male and one female), determined by a challenge in which the winner got the partner of their choice.
Split into simpler sentences: At the beginning of the game, the 12 kids are paired off into six teams of 2 (each composed of one male and one female). This is determined by a challenge in which the winner gets the partner of their choice.