My two cents is that this is just the new normal. Amidst all the takes here (and ngl, some of which are quite incelly), fact remains that even if China is a middle income country, the standard of living in most of the big cities is now comparable to that of developed countries and the young likewise increasingly have the same mindset.
This isn't the middle ages anymore, where you basically needed children otherwise there'd be nobody to till your farm when you grow old and frail. People have children mostly for the novelty, and yes some still have children as a retirement plan. But whose to say with automation progressing the way it is, retirement homes will be so advanced that you won't even need children?
You can push incentives for people to have children, but as with anything in China enforcement will be scattershot, and by and large people will just ignore the rules especially if the penalties are simply economic rather than being thrown in jail. Now if we get to the latter part in terms of enforcement the predictable will happen, you'll just get brain drain. You could promote a culture that glorifies having children, but that takes time and considering the Chinese entertainment industry is still in its infancy and also hit a road bump because of COVID and government crackdowns, don't count on that happening anytime soon.
In terms of solutions, I think China should seriously start considering a temporary migrant worker program the same way the Gulf countries do, especially since I know not all Chinese are privy to the idea of immigration nor do I want the social chaos you see in Western countries. Come to think of it, I don't know why this should be a problem, since many African countries are already implementing Chinese education in their curriculum. Shit from online youtube vids, many African teens already speak Chinese way better than I do. There's also the much discussed fact that automation will mean countries are going to need smaller work forces in the future.
More to the point broadly, and I guess this is the case with all countries. Socieites that have the demographic dividend now will meet the same fate once they get close to industrialized status. So perhaps its time to search for a new economic model that won't be so reliant on how many babies a country can pop out.