"forbid poor people from having children" - OP

That is essentially the eugenicist argument. A corollary could just as easily be, 'forbid rich people from having children' - so their wealth can be utilized to bring public health, education, opportunities, food security, to all people. Plus, it would take fewer people to achieve the objective, too.

Instead, how about raising the floor for all people, across the globe. Giving health, agency and opportunity to all people in lieu of preserving wealth accumulation by the few. If people had food, shelter security, decent living standards, they would, IMHO, be better off. We would have a vibrant global society, raise well-adjusted children and in general, do the right and healthy thing.