Tell them it's much more heartless to have kids when you know you don't want them

Don't worry. I hate kids too. I do however, love my own. It is very different when you have to deal with other peoples kids.

I'd be cool with that. My mom has always called me evil and heartless for hating kids or family and I've called her weak for letting my sister's kids/and the kids at work walk all over her. She did that for years until she finally started realizing how relaxed I am compared to how stressed she is. You know your temperament, your limitations, and abilities. If you don't want kids don't have them. It's you who will have to deal with them mostly. Not them.

At first I thought it said "chicken" instead of children and I got really confused.

Great, than I'm heartless too. I just don't like little kids.

although they are entitled to their own opinion, in the end it is the decision of yourself and your spouse. keep your chin up and do what you feel is best!!!

do they not understand the world is overpopulated?

I don't care if you don't want to have children, but you don't have to be mean and say you hate children. Keep it to yourself. After all, you were a child once, OP.

And how are we going to stave off the planet's desperate underpopulation? ?

If your parents want grandkids they should just adopt a little kid and pretend it their grandkid.