The candidate requirements:
About visa:
The type of visa depends on the length of time the au pair candidate would like to stay in the country. To stay longer than 6 months, the au pair must apply for a student visa, and enroll in a course at a Chinese University, language school, or any course that is not a short-term.
To apply for the visa (independent of the type of the visa) the au pair candidate has to have:
Invitation letter:
The au pair candidate needs this letter before applying for the visa. The Chinese government requires the au pair candidate to present this letter. It can be issued by an au pair agency registered in China or by the host family. The letter must contain the following information:
Info bonus:
1- Visa processing time is 4 business days;
2- A student visa does not allow the candidate to work in China;
3- If the au pair candidate wishes to leave the country during your exchange program, or visit Macau and Hong Kong, he or she will need at least a double entry visa.
Source: https://www.aupair.com/pt/p-china-requerimentos.php