Assuming your son in born in UK.
Ignore the grandparent’s details. You only need to send your original ILR BRP and your son’s original UK birth certificate. Add your original passport also if the checklist asks.
I used the following cover letter for my daughter in a similar situation, you may use it too.
To,
Her Majesty’s Passport Office
RE: First Child British Passport Application
Child Applicant: [Name] [DOB]
Application Reference: PEX XX
Dear Sir/Madam,
I, XX am the mother of the above named child applicant. I have Indefinite Leave to Remain in UK i.e. settled status since DATE XX.
Please refer to:
British Nationality Act 1981
1981 c. 61 Part 1 Acquisition after Commencement Section 1
Acquisition by birth or adoption
(1) A person born in the United Kingdom after commencement shall be a British citizen if at the time of the birth his father or mother is—
(a) A British citizen; or
(b) Settled in the United Kingdom
According to the above provision in law, my child is British by birth; at the time of my child’s birth, I (mother) had settled status in the United Kingdom.
Please find enclosed the following original documents:
• Original Indefinite Leave to Remain Biometric Resident Permit Card - XXXXX (mother’s settled status in UK)
• Original Child’s Full UK Birth Certificate
I believe the above documents are enough to prove my child’s claim to British Nationality.
Please feel free to contact me if any additional documents are required.
Thank you,
Kind Regards,
Name XX
(Mother of the child)
Address:
Email:
Mobile:
If you are the father applying, add your marriage certificate and mother’s consent also. If you are the mother applying, no need to.
NOTE: you should retain your ILR BRP even after naturalisation as it is the proof for child’s British Citizenship. Do not return it to HO even after naturalisation.