Divide the children into three groups. The first group devise theme A - perhaps on metallophones, keyboards, strings or recorders - to express the calm and order of good digestion. Use smooth rhythms and melodies based on scales.
Group two are the worms and could use short, fast, contrasting phrases of "wriggling" chromatic motifs, disturbing the gentle patterns of theme A.
Group three, the leaves, should create a line with an appropriately rough surface texture, full of large spiky melodic leaps and unexpected percussive intrusions.
Play the piece in the order A AB ABC AB A and not only have you told a story of the transition from digestive tranquility to parasitic disturbance and back again, you'll have used a symmetrical musical form found in Haydn and Mozart.