Why do Buddhists have children?
I think you mix up pain with suffering.
Pain is something that happens. Suffering you do to yourself, a second arrow

And from what I understand Buddhists view being born human to be lucky, as you get the chance to have influence on your own fate. You can get the chance to hear about and perhaps understand the Four Noble Truths and start following the Eightfold Path that leads to an end of suffering (but not pain(1)). And perhaps even an escape to the cycle of rebirth.

So Buddhists don't really have a reason to not have children.

(1)Be happy, you have done the first steps.