เพิ่งส่งe-mailไปถามสถานฑูตมาค่ะ (น่าจะถามสถานฑูตมากกว่าPantipนะคะ) ได้คำตอบมาแบบนี้ค่ะ ลูกสาวจำเป็นต้องอยู่เกินเหมือนกันค่ะ

Dear Sir/Madam,
  
Thank you for your email.
  
J visa holders have 30 days following the successful completion of the exchange visitor’s program to be used for domestic travel and/or to prepare for and depart from the United States. Upon completion of your exchange program, you have a grace period of 30 days to depart the United States.

Please find more information regarding the 30-day grace period under ‘Exiting the Program’ in the following link:
https://j1visa.state.gov/participants/common-questions/

Regards,
Nonimmigrant Visa Unit
U.S. Embassy Bangkok