While initially fun (because she clearly stated that she wanted only 3 kids as a personal choice) this joke had become cheesy and hurtful for her. She was actually starting to feel guilty because they were so insistant.
And this situation sucked because she's a great coworker, a great mum and she didn't deserve to feel guilty. Especially since no-one really cared in truth, it was just "a joke". So I told my boss and he stoped quickly and now everybody involved feel better. He was just sincerely unaware of the impact of theses recurring jokes.
So yes of course you can make joke but it's just critical to be aware that you can sometime unwillingly hurt people. And when that happen you should just stop this joke and maybe even apologize (depending on how far you went before you realize your joke wasn't funny).