Depends

Family green card application has two steps

(1) I-130 application by the US citizen sponsor 

(2) application by the immigrant

if the immigrant is outside US, they needs to go to local US consulate, this is called consulate process.

if the immigrant is inside US, they needs to file I-485 adjustment of status. 

So if you are still in the I-130 application by their travel date, your parents can come to US on an exisiting B1/B2 visa.  If they have no visa and needs to apply for a new visitor visa in Aug, it might be difficult due to the I-130 application. 

However, if your I-130 application is approved and their file being transferred to US consulate in China, I'd suggest they wait in China. 

In either case, they need to prepare a lot of paperwork such as police certificate and birth certificate, it is best they are in China to get the paperwork rather than sitting in US.  I would go the consulate process rather than 485 route, if your parents are healthy and mobile.