I don't suggest the parent to enter US to file I-485 using B-1 visa because USCIS may consider it is the visa fault. The parent can apply for immigration visa in China, and then enter US to become a green card holder. After that, the child can file I-485 within US after the parent becomes a green card holder.
By the way, the child CSPA age is temporarily frozen when the I-130 PD is current, and the child can file I-485 within one year to freeze the CSPA age permanently even if teh CSPA age is > 21 when he/she files I-485. So, as long as teh parent gets the immigration visa within one year to enter US from the date when I-130 PD is current, the the child can file I-485.
Another choice is that the child can go to China to get the immigtration visa together with the parent.
