The average environmental impact of even poor people in rich nations is many times higher than even rich people in poor nations.

It's often around 1t CO~2~e for a poor person in developing country vs. 5-10t CO~2~e for a poor person in an industrialized country.

However, rich people in Western countries tend to be in the 100s or 1000s of tons of CO~2~e/p/y which is extremely far off from being sustainable.

But I want to emphasize that this is just the current state. How your child lives in 20 or 30 years, you don't know. It may use much fewer resources or much more. I am cautiously optimistic that they will use fewer resources than we do. The question is more whether it will be enough.

a) Having fewer kids is extremely environmentally friendly, in any nation, and especially the West

1t CO~2~e/person/year is roughly sustainable within the current ecosystem. Thus, many people in poor countries are at or near climate neutrality already. If people live sustainably already, then no, there is no inherent need to reduce population or necessarily have fewer children.

That's not to say there may not be other benefits to having fewer children.

Each child produces around 60x the CO2 offset by one person going vegan for life.

Again, this is true only in the current situation and in Western countries.

b) Migration from poor nations to rich nations is extremely damaging to the environment. Consumption matches Western patterns almost immediately.

Blaming CO~2~e emissions on migrants is a bit disingenuous. But if it helps you make the case to yourself that Western countries should do more to give people in developing nations safer lives so they don't have to flee, I guess I'll take it.