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What ideas have you thought of yourself so far? You need to research current issues with sustainability so you can attempt to design a solution. Doing it for children is an interesting angle, make sure you make it as interactive and fun as possible as otherwise they wont be interested in it. This doesnt have to be directly 'teaching' either, i saw an example of a rubbish bin that had a sort of chute slide and was in the shape of a giraffe, i.e getting children to not drop their litter in a more fun interactive way.