My sister in law is coming to visit us in March from the UK, with her 6 year old son. She has read that as she is a lone parent travelling with a child she may be required to produce 'further documentary evidence' to prove that she is related to the child - apparently the passport alone is not sufficient.

I have tried to research this on the internet but cannot find anything which mentions it.

There must be people on this website who have travelled to and from the UK as a lone parent with their child. Has anyone encountered problems entering China or know anything about what this might be referring to.
Thanks for any help.