I find this often expressed opinion rather chilling; I have a gut reaction against social engineering, though I do understand the problem that exponents of the idea are trying to solve.
First and foremost, the maxim doesn't proffer any remedy for children that are already born, and that is the main problem in hand. How do we ensure that their lives turn out to be worthwewhile and fulfilling?
How do people come to be poor? Do they choose poverty? Poor people are often characterised as lazy. Perhaps some are, but there are certainly many that aren't. So if they are not poor by choice, they are probably poor because their parents were poor or because they lack the capacity to hold down a well-paid job (or both). Middle class people make informed decisions about how many children to have. They can make informed decisions because they are better educated. Therefore if poor peope were better educated, they too would make better or more informed decisions. But do they have the intellectual capacity to take on this education? If they don't, should they also be blamed for it?
It seems to me that we have a socio-economic system that on the one hand creates or allows poverty and on the other admonishes people for not dealing with that poverty by choosing to limit their reproduction. If poor people don't have so many children, then fewer potentially ill-educated people will come into the world, but crucially, unless innate intelligence is hereditary, so will fewer intelligent yet poor people.
The instinct to reproduce must surely be about the most innate characteristic that humans (like any creature) have. There may be people that don't grasp the long term consequences of their reproduction, but are there seriously any that reproduce because they think they will be financially better off? [OK maybe some women get pregnant to get a coucil property, but that aside].
If better off people have fewer children, then one way of ensuring poor people have fewer children is to make them better off, surely? But tjose who don't want to subsidise poor people with chilldren presumably don't want to subsidise them to not have children. They don't want to subsidise them full stop. You're poor, you shouild stay poor, and if you place yourself in a worse position by your reproduction, don't come crying to me.
Like any species, we should increase our population. It follows that any new born should be cared for so that s/he can turn into a net ccontributor to society in later life. Society should invest in this. It isn't easy but it is also not something we should shy away from. The basic building blocks of nutrition/water and protection from environmental extremes should be easy to provide, and an excellent start.