If your wife and kid come back before 8/31, they can apply for visa holding their current original I-797. Otherwise, they have to wait until your (their) new I-797. If they they have to go back immediately and come back after 8/31, make an appointment on the day they plan to come back. If your new I-797 is approved before the interview date, mail to them by Fedx (3 days). Otherwise, cancel teh interview date and make another appointment. It sounds a little bit complicated and good luck.