Historically, poor people's children started being "profitable" (as in doing more work than they take) much more quickly - they were raised communally (spreading the required work evenly), and they started working and helping earlier. This made having many children a legitimate strategy to deal with poverty. This has completely turned around - community is almost non-existent, and children will really only be "profitable" once they are adults.

The situations are simply too different for your argument to make sense.

Fair point, but I think you also overestimate how many people make decisions using actual cost-benefit analysis.