The population doesn't just go from 100m to 50m instantly. It gradually changes over time, 100 years from now the smaller population will work itself out, but none of us will be alive for that. We still have 100 years of discomfort to get through.

Fewer children mean all the industries and gov't services who are employed now to service children will need to downsize, these are lost jobs now before the fewer children grow to adults where they would take over those fewer jobs. All of this will have a effect across the economy.

Pediatricians, Teachers, Toys & Games companies, Children Furniture, School Supplies, Electronics, etc.... All of these are sized with the expectation of the same consumer demand, but when there are less kids to buy and service each of these will be forced to downsize. Again in the long run it works out, but in the short run say next 50 years for people in these markets will see downsizing over time. Can the rest of the economies pick that up?