WHAT'S "WRONG" WITH YOU THAT YOU DON'T HAVE CHILDREN? NOTHING!
First tactic. Turn the question around!
"What's wrong with you that... you have children? You have many children (that you can't afford to give them a decent life)? What's wrong with you that you not wait until you were married when you could not afford children / give them what they deserve / provide a family for them?" "What's wrong with you that you did not use contraception when there are so many options these days?"
"What's wrong with you for asking? "
You get the idea. I would love to hear some real life RETORTS!