My impression, and of course I may have the wrong impression, is that this is very much about the parents since they seemingly removed their elder child for similar reasons, and the BBC article notes: 'The Rowes say the suggestion that gender is fluid conflicts with their Christian beliefs and they are seeking a legal challenge against the school's actions.' and given the current publicity, which I understand saw them giving a TV interview and have the support of the Christian Legal Centre, they clearly don't see this as a private matter.
In my experience (and I have some) 6 year old children are easily confused and aren't able to handle issues like gender and, say, death but I'd imagine most parents find ways to provide appropriate age-related reassurance: I'd imagine many parents will do this, say, when the hamster bought for a birthday present decides to expire after less than a week. Perhaps they never thought of this approach before making what should have been a private family matter between themselves and the school into a public one involving their religious allegiance, which is perhaps the downside of having schools with a defined religious ethos.
Will follow events with interest.
