Cabbage White Caterpillars

http://www.thebutterflysite.com/rearing.shtml

thought of keeping one and let the kids watch it develop into a butterfly, as long as they dont get upset if it dies of course

 

if you're planning on giving it a go it wouldn't be a bad idea to start with a number of them as opposed to just the one so the kids wont notice if the odd one goes to the big cabbage patch in the sky

or...eh...explodes with parasitic wasp larva