The thing is that, after you decide to not have kids, time keeps up it's inexorable march. The earth has a carrying capacity for humans. We haven't hit it yet. But we will. This is the one absolute of life: species expand in population until there's no more niche left to occupy.
So after you decide not to have children, other people will have children. Eventually, their descendents will take up the space in the earth's carrying capacity that your descendants would have taken up. There is nothing you can do to stop this. So the best strategy is to change the proportion of people who have an environmentalist bent. Children are an effective way to accomplish that.
In addition to worrying about the carbon footprint of another kid, I have a much bigger concern that
* We will do nothing regarding climate change (seems likely)
* The predictions for Business as Usual will have been too conservative (seems likely given that the Arctic is melting well ahead of schedule, and who knows how clathrates pan out)
* Temps rise by a good 8C or so in the next 8 decades
* My child, or their children, will die horribly in a new global Holodomor as our resource chains (webs, really) collapse and the world's civilizations implode.
Anyway, I already had one. But I have a deep, pervasive dread that I have doomed them to the above. However, I also might be wrong! Maybe it just gets a bit warmer and they can open a nice winery in Sweden and it's all fine.
Let your kids worry about that. I know I, personally, would rather be alive and facing catastrophe than not be alive. And history is filled with examples of people who persevered through all kinds of calamity. We're a resourceful bunch. And it'll be easier for your children to face the future together than for your child to face it alone.
So after you decide not to have children, other people will have children. Eventually, their descendents will take up the space in the earth's carrying capacity that your descendants would have taken up. There is nothing you can do to stop this. So the best strategy is to change the proportion of people who have an environmentalist bent. Children are an effective way to accomplish that.