"It is not ethical to tell people that they should have less children."
Factually, the earth does not care about ethics. Too many people consume too much resources. Once resources are depleted, nature will restore the balance the hard way. People will die of famines and lack of water.
This is exactly the same in poor countries as in rich countries, for black, white or yellow people. More children require more land, more schools, more food, more clothes, more water, more money.
The rich countries obfuscate the fact that population increase is depleting the environment because they can still import resources (food, water, and minerals) and they still have the money to construct houses and schools for a growing population. But the rich live in the same world as the poor. Therefore increasing resource scarcities will sooner or later be felt everywhere.
Having fewer children means living "lighter" on the earth, using less resources, saving more future.
Having many children may have meant happiness and a guarantee for old age in the past.
In the present, "many children" means no guarantee for retirement. On the contrary. Since many children are not finding space and land and work in their home area, they are forced to wander off into the cities, mostly ending up in situations worse than their parents had at home, when they were young.
It is true that a baby born in a rich country weighs far heavier on the earth than one that's born to a poor family.
But as long as distribution of resources remains unequal, the poor will be better off if their labour has to feed fewer offspring. The rich have already reduced their number of children because it leaves them with more money for a better life-style.