回複：父母探親簽證一年有效期和再次入境問題

First, you should check when their B-1/2 status expires. It may be shorter than one year. If their status expires in six months, they have to leave the U.S. in October unless filing for extension.
Assuming the B1/2 status will not expire before they leave the country. As long as your parents come back before the visa expiration date, they won't have any problem coming back. However, how long they can stay in the U.S. is another issue. They'd better prove that their stay will be temporary and also may need to show their return tickets have been booked in six months. Bottom line, it is up to CBP officer to decide how long they can stay.