The first contact most children have with death is when a beloved pet dies. As it is unpleasant to watch the changes in a carcass as it decays, you can give them some introduction to how materials are recycled in nature by observing the changes in a fruit such as a tomato, orange or apple. Put bits of fruit in a clean screw-topped jar. Seal and watch what happens over the next two to three weeks. There will be a succession of fungal growth as the fruit rots.
In China, they import the wood of the Taiwanese coffin tree (Taiwania cryptomerioides) for making coffins because its resinous wood has preservative qualities. How do we exploit wood? There's a specimen in the Temperate House at Kew Gardens and illustrations and information about the coffin tree's vulnerable status can be found at www.botanik.uni-bonn.deconiferscutai
Sexton beetles (there are several species in the UK) bury dead animals to provide food for their young; they find the carcasses using strangely shaped antennae. See www.biopix.dk and www.kendall-bioresearch.co.uk
Blow-fly larvae are used to "age" a corpse or provide other evidence in forensic science. Research, using the good summary diagram, can be found at http:agspsrv34.agric.wa.gov.auentoforensic.htm and information at www.fathom.comfeature122347. How long does it take for blow-fly eggs to hatch or for the maggots to complete the various stages of their life cycle?