一個可能幼稚的問題: Returning Resident Visa容易申請嗎？
-As long as you have the strong evidences that you can not return to US in time due to the factor uncontrolled by you, you should be OK to get the SB-1 visa.
See these websites for more information on SB-1 visa:
http://travel.state.gov/visa/immigrants/info/info_1333.html
http://www.immihelp.com/greencard/returning-resident-visa.html
Also you can get the transportation letter for your child when you apply for the SB-1 visa.
另外，如果現在（一年的期限到期之前）給使館寫信, 說明情況和繼續PERMANENT RESIDENT的INTENTION, 會有幫助嗎?
-Not hurt you to try, but it might not be useful.
