ethnic genocide to help reduce climate change is quite different from not having children, because the latter entails literally no suffering, in fact, any notion of suffering of an unexisting human is meaningless and void
a good analogy would be choosing to abort children with genetic defects, because they will suffer their entire lives and/or possibly need things that would then be unavailable to already existing humans like donors' organs, medical help etc, which isn't considered eugenics
also, eco fascism, compared to not having a child in a developed country, is completely impractical (i feel pretty cruel saying this, but i need to somehow stood down to their level to argue against it from a practical point of view), because people in developing and poorer countries typically have the least environmental impact...
the childfree and vegan movement need to tread carefully to not fall into eco-fascism or eugenics, in the sense that people should not be physically prevented from having children and we shouldn't kill non-vegans, but they should at the same time be aware of what having a child means for you, for them, animals and the earth
more non-vegan people definitely brings more animal suffering, not sure what to add here