Slowing Population Growth without restricting freedom of reproduction
Pretty simple really:
Improve the standard of living in as many places as we can (the population growth rate will decline)
Empower women so that they are the equal of men in as many ways as possible, including full control of their bodies and reproductive choices in as many places possible.
We know enough about demographics to know that when you do these things, the birth rates decline.
What I don't support and what you shouldn't either is:
Reducing support to children and parents of young children to reduce the birth rate.
The reason is that birth rates tend to be higher among those who are poorer, all things being equal, so reducing support or benefits to those with children takes away from children who suffer more poverty than any other age group (at least in this country). And no level of poverty is acceptable, but especially not where children are concerned.
The children in the US who live in poverty already battle lack of nutrition, homelessness, lack of health care. Many children have the basics because of the Earned Income Tax credit and other programs that help children by helping their parents. If you take away those things, you harm children directly. If you want to take away those things to slow population growth, you are harming children in poverty NOW to accomplish what you think will make the world a better place.
But that would not be helping, that would be harming. You don't start saving the world by targeting a child from whom you will deprive of something to make the world a better place.