That is the case that parents are permanent residents and baby can get only permenent residency at POE, not citizenship. Your baby is a US citizen.
http://www.uscis.gov/portal/site/us...nnel=063807b03d92b010VgnVCM10000045f3d6a1RCRD
1. Does my child qualify for automatic citizenship under the CCA?
Under the CCA, your child will automatically acquire U.S. citizenship on the date that all of the following requirements are satisfied:
*
At least one parent is a U.S. citizen,
* The child is under 18 years of age, and
* The child is admitted to the United States as an immigrant.
Follow what DOS site says.