We have a client who has a 7 yr old son living with her but who has only just come over from Africa, brought by another relative on their papers. The relative has since returned. Unfortunately she has no documentation for this child, no birth certificate or anything.

Some time ago she applied for Child Benefit but she appears to have been refused because of the lack of docs. What happens in these circs? IR can ask for any documents 'considered relevant to your claim' but what if no docs exist? Is she left with no CB/CTC forever?

Any tips please?

Take a look at the DWP leaflet GL25 "How to prove your identity for social security". A pdf version is available at http://www.asylumsupport.org.uk/docs/indentity.pdf

(For some reason only part of my posting was shown.)

Use the link as printed; it really does spell "identity" wrong.
I think that the guardian can also make a statutory declaration at a solicitors which can serve as "evidence" of date of birth.

If she is applying to regularise the child's stay in the UK, she will probably have to provide some evidence of identity for the Home Office. Her immigration solicitor (if she has one) might be addressing this issue also, and might be able to help.