Parental responsibility documents for FLR M extension

Hi
I have two kids. One will be applying with me for FLRM extension other is British citizen. I understand that I have to provide birth certificate and travel document for both. Which documents prove parental responsibility? Tax credit letter?
And what other documents do I need to provide for the child who is already BRitish. Thankyou

Full birth certificate will prove parental responsibility and anything else you have received through the courts if you ever entered court.
To prove your British child would be to provide full birth certificate and fathers passport or fathers British birth certificate along with a proof of address for him.