What happens if only 10 to 20% of us have children.
Well for a start the population will decline. This will take a generation to clear through the issues this will create such as covering the pension burden for example requiring older people to sell their homes to pay for their underfunded pensions etc.
However the upside will be a massive environmental improvement- less people means less CO2, less plastics, less all environmental damage.
It will also mean that there are more natural resources to share amongst fewer people. Less pressure on infrastructure (water, roads etc). And companies will have to pay higher wages to attract the fewer workers (see what happened to farm workers wages and conditions after the Black Death). But in short fewer people will create better economic conditions for those fewer people.
This is what has been happening in Japan where they have gone through about 30 years of stagnation dealing with an ageing population but are now starting to come out the other side with a stronger economy and lower environmental impact.