Supporting Docs for Childs First Passport

Dear Community members

We as a family are all ILR holders and by virtue of that my youngest child who was born in UK has been granted UK citizenship. Next step is applying for his passport. In a haste I applied using the online service as opposed to the Post Office Check and Send. Now I need some clarity as to what needs to accompany his application. As I understand it is the following:
1. Application print with counter-signatory
2. 2 x photogrpahs
3. Naturalisation certificate
4. His current non-UK passport

There are certain other sites/docs which seems to indicate that the passport and ILR of one of the parent also needs to be provided. Some suggests the marraiges certificate too. It is rather unclear. I would appreciate if anyone has experience on this and can comment/clarify.

Thanks in advance.