I think one of the points I've picked up from conversations I've had with settings that have been through an inspection, from Ofsted themselves and also my own experience when I was teaching (but under previous framework) is, whilst there is no need to have anything written down/recorded, staff must be able to talk clearly about the children, the provision put in place for them, and also why you have planned those next steps for them.
If a conversation between the member of staff and inspector doesn't answer the questions clearly or is muddled, then the inspection team may want to dig deeper into this, and this is when having something written down may help. It shouldn't be a requirement though.
The main issue is though, when an inspector asks someone a question, panic can set in and their thinking might not be as clear as if it was when they were just having a conversation with other staff. This can (unfairly) make it sound like they don't know their children. Also, as @louby loo mentions above, if the member of staff is away, what systems do you have in place to ensure that their key children aren't missed?
It certainly shouldn't be anything too detailed as it's just the backup and shows that you've thought about it.