I'm going to snap at a co-worker for assuming that I don't have problems if I don't have kids
Why are you going to snap at her about it?
She just assumes that if I don't have kids I don't have any problems.
I'm sorry that this person was so rude to you. I don't think you should snap at the person for things that she assumes about you. Making ignorant statements about other people's lives is rude.
I'm going to make her miserable because of my hurt feelings.
Stating those kind of assumptions she did was not very cool of her. Snapping at her is just not really going to change it any, so I would suggest to talk it over and be civil towards her. Maybe she will realize she was wrong, what have you got to lose?