They're playing musical chairs, and when the music stops they all sit down and this child missed out on the last chair so he has to be excluded from the game until it is finished.
This could be any number of things.
1) Are they playing a game?
2) Are they having a competition?
3) Are they doing an exercise? ie. People with two syllables in their surname go over here, people with 3 go over here, things like that)
4) Did the boy do something wrong and thats why he obeyed?