I think it's silly and actually a little ignorant to not like people just because they are young. I've met plenty of good kids in my lifetime that were more than pleasant to talk to and be around, I've met some litte hellions too. But then, I've met nice people and jerks of almost EVERY age bracket.
When most people say they don't like kids, they are usually talking about the brats of the world, but lest we forget there are plenty if not more adult jerks in the world, so I think it's just easier to say that you hate ungrateful jerks of any age, rather than arbitrarily singling out children. People do it because they are lazy and uninformed, or feel awkward around children. I find in my experience, those who bark on about how bad children are, are usually the LAST to do anything about it, if put into a situation where they really should.