Yes, they may apply for a multi-entry visa from the Chinese embassy where your wife and son live.
As for the documents, your wife have to provide the followings normally:
1. Her and your son's passports with blank pages and at least 6 months validity left before expiration.
2.A truly and completely filled application form affixed with one passport-sized photo.
3.A Visa Notification Form issued by Chinese
authorized unit.
4. An invitation letter from you.
5. the photocopy of your marriage certificate, a notary certificate of kinship etc, your son's Birth Certificate.
As for the visa fee, it depends on your wife's nationality.