Interestingly, we were just discussing the fact that children seem to be genetically programmed to dislike certain foods and speculated it was a survival mechanism to protect against dangerous toxins and food poisoning.
We also wondered if getting kids to eat their greens wasn't a bit of a waste of time, therefore, and not worth losing too much sleep over. There's a good chance that their little bodies aren't as desperately in need of the vitamins that leafy green veg provides as older folk. Getting them to eat fruit sounds far less stressful for the parents.