My 2 and a half year old is not a very good eater, and I definitely find it difficult to get him to eat enough fruit & veg.
Recently I bought some Food Doctor - seed mix from an organic health food store thinking I might try and sneak some into there cooked meals. However, DS2 loves eating it raw - he asks "seed mix" and can actually be diverted from biscuit requests by the promise of seed mix.
its fab stuff - packed full of omega oils, protein - giving loads of energy without all those ups and down that sweet things create. I would say that his behaviour has improved, he is calmer and much more biddable.