baby born in US,parents Chinese citizens. Mom wants to take her to China-what papers are needed?

Do you mean you will go to US to pick up your baby? I think a tourist visa will be ok for you, and you have to prepare the following documents:
1. Valid Passport with at least 6 months validity.
2. The DS-160 application form confirmation page.
3. One photograph of two inches square (51mm x 51mm) color photograph, less than 6 months old, against a white background, full frontal view.
4. Original Bank Receipt for Application Fee.
5. Passports containing all previous U.S. visas, even if expired.
6. Evidence of Why You Will Return to China.
7. Invitation Letter
8. Proof of Finances
9. proof that you have a genuine, ongoing relationship with the person inviting you