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.

I lived with my dad from January to October and then I moved out on my own in October. The request letter asks for documentation showing where my daughter lived and for how long at each address. Do I need something official, or can I write a letter and sign it? I'm not sure what kind of document can prove a 2-year old's residency for EITC.