Can mothers become Japanese citizens and children become Japanese citizens?

According to the provisions of international law, babies born in any country, regardless of the nationality of their parents, have the nationality of their birthplace. As you said, there is no general coverage in international law, and different countries have different provisions in their nationality laws. China's regulations are OK, while Japanese immigration policy is relatively loose. If the mother is Japanese, the procedure should be much simpler!

Also, China is a country with a single nationality, so if you want to join Japanese nationality, you will automatically give up your China nationality!