The latter seems to be solving itself, as birth rates are steadily on the decline. Though it tends to happen in conjunction with development (and thus more intensive energy usage).

In general...where there are 5+ children there is very little hope; there is extreme poverty (read: little consumption); and so on.

Children don't survive in those environments. Higher birthrates is a family's / culture's answer to that. It's playing the odds.

Put another way, if there was consumption to Western middle class levels, there would be no need to have 5+ children. This is why, as a middle class grows, birth rates fall.

Then we should find the optimal point of socio-economic development with regards to co2 emissions? Or is upward mobility impossible to remove from the system?