You need to ask lawyer in China to see what Chinese law says,

regarding adopt other's kid (i.e., the kid from previous marriage).  From my understanding, in US, if biological parents are still alive, both of them have to agree for the kid to be adopted.  Without the signature from both parents, the kid cannot even get a passport.

(I feel, in China, the mom might be able to get passport and visa for the child without the father knows. I do not think this is right, though.)