That's a good question. At first glance, per-captita consumption seems like the thing to focus on. Honestly, it's probably a good place to star; however, as that gap closes things get more subtle.
Consider 4 people, entitled to equal carbon emissions. They form 2 couples and the first decides to have 2 kids, while the second decides to have 6. Are the resulting families entitled to equal emissions, or is each person still entitled to equal emissions?
- It's unfair that children born into a large family should have less then those born into a small family
- It's also unfair that the actions of a couple choosing to have a large family should slice the environmental pie more thinly, reducing the share of those who chose to have a small family. Furthermore, the first option leads to the question of what's a fair baseline and basic human rights, while the second leads to a tragedy of the commons or lowest-common-denominator scenario.