Give every child an instrument, making sure there are many contrasting timbres. Blow a single large bubble and ask them to play when, and only when, the bubble bursts. After a little practice, you will get a multiple sforzando chord. Then apply the opposite rule: play all the time the bubble is in the air, and stop playing the moment it pops. This time, you'll get sustained sounds ending in sudden, seemingly audible, silence. Then blow a stream of bubbles and repeat the two activities, asking each child silently to choose one bubble to be their "conductor". For the first activity, you'll now get a series of irregularly punctuated chords of varying dynamics. For the second, you'll get complex textures that dwindle into silence.
Help the children to suggest new rules: eg, wooden instruments play the short percussive "pop" sound, metal ones play the long resonant sound, rattling instruments play either. Listen to some of the marvellous compositions of the late Witold Lutoslawski, such as the three "Chain" pieces or the 3rd Symphony, which incorporate similar principles.