There is nothing you can do to reduce your carbon footprint that comes even close to having no or fewer children.
You can get rid of your car, stop eating meat, recycle as much as you can and so on, but the impact of that together is a fraction of the reduction you can achieve by simply not creampieing your gf/wife.
You’ll also save a ton of money and you don’t have to deal with any annoying kids so it’s a win/win all around.