General rule, foreign nationals need a visa to visit Mainland China. There are exceptions to the general rule when a visa-free entry can be granted. But these will not apply in such circumstances for a visiting foreign national. It is important to note that although officially part of China, Hong Kong and Macau will have visa-free entry for most foreign nationals.
The visa type you are going to apply is an ‘F’ visa which is for culture exchange. Currently, there are no specific visas for the classification of an au pair in China. An F visa is for all culture exchange purpose. However, each embassy may have their own interpretation on the type of visa that should be applied. For example, a recent Australian candidate received an X2 visa for short-term study.