I've sometimes used the game Simon Says on young children. You probably know the game: the children have to do everything Simon says, but nothing else:
"Simon says, 'Stand up.'" (Children stand up)
"Simon says, 'Clap your hands.'" (Children clap hands)
"Jump up and down." (Children do nothing, because Simon didn't say)
It's fun trying to catch the children out, and over the weeks you can gradually increase the repertoire of commands they should (or should not) follow.