It's probably too late for population reductions to save us. If we want to stay below two degrees of warming, massive change needs to happen over the next fifteen years. Not having children, or delaying children can only help a little with this. You won't see a 90% reduction of the population in that timeframe, even if nobody had kids. Keeping population levels approximately constant or falling is important in the long term, but the climate catastrophe is by now a short term problem.

The only realistic way is a huge investment in renewable energy and storage, paired with a steep price on carbon emissions.