How much monthly finance is enough to support a family with 1 kid

It depends on you...wherever you live in China...if you live like a "westerner" it is very expensive to live in China, if you live like a local it can be very cheap.  As a rule of thumb, you will live within your means, as a relative lifestyle, living within your means, you will live better in China when compared to most foreign cities. 

Although, the cost of living here is constantly rising...it is still not at the level of most US and European cities.  Many foreigners come here though and want to live like a European or American in China (i.e. eat at western restaurants, drive European cars, live in large villas, wear western clothing) China is then extraordinarily expensive to live in but, if you get a moderate apartment, take the metro most places and eat at local restaurants and shop at the local malls, it is inexpensive to reasonable to live here.  So, in summary, how you will live depends on YOU!