How do I prove residency for my 2-year old for the year of 2020? The FTB is requesting additional info, and one document they want is a proof of residency for the child.

That's odd.  I have never heard of the IRS questioning a tax year before the due date for that year hasn't even passed.  Are you sure it is not 2019 they are asking about?

What it the code on the letter - is the a CPxxx or LTR-xxx?

So I just realized, the letter is from the Franchise Tax Board, and I think because I filed for earned income tax credit, they want additional info to approve it my state tax return. I don't see any CPxxx or LTR-xxx codes on the letter. Just FTB4502.

Sounds like your father might have also claimed the EIC.   Only one taxpayer can claim it.   CA is real picky.

It should not matter what address you lived at as ling as you child lived with you all year the address should be irrelevant.  I would call the FTB and find out what they want.

I will call in the morning, I just don't know what kind of document I can get with name on it to prove her residency, or how to get it really.

Doctor bills perhaps.

I just got this same letter for my two year old. We don’t have any doctor bills. Ugh 

I ended up getting her pediatrician to write and sign a letter on a letterhead with her name, DOB, and home address specifying she lived there with me (including my name, DOB and address) from Jan 1-Dec31 with the year.