Ok. I have an update on my thread hoping that it will be useful for othe Sajhaians.
My parents were granted multiple entry visa sticker (I understand the actual visa is granted a POE).
I provided the following docs-
1. Cover Letter to consulate -mentioneing the reason as to join with family and celebrate arrival of new baby
2. Employment Verification letter
3. Doc's letter regarding expected delivery date 
4. One year tax return
5. one pay stub
6. copy of green card
7. copy of proof of relationship (birthe certificate)
8. Affidavit of Support
9. One bank statement
 
Thank you guys.