My thank you letter tells me we qualified for the child tax credit, however the 1040 form has "credit for other dependent" why are they different?

Did you put down the baby lived with you the whole year?  

A child born (even on Dec 31) is considered to live with you the whole year.  So pick the Whole Year at the top of the drop down box.

I discovered this is my issue, when asked how many months they lived with me, I input how many months they have been alive... there wasnt much direction on if they were born this year.  So.... now i think since my return has been accepted, i need to amend the return, which would eventually give me a larger return.....