When my son was 10 I ran through some role playing with him regarding interviews at senior schools. Also, some parents asked me to to do the same with their sons, these boys were down for Eton, Harrow, Charterhouse etc. It is amazing how a practise run can sharpen things up.
For example, I asked one boy why he wanted to go to Wellington College and he said because his parents told him so! Also, things like holding out your hand for a handshake and not making it too limp. I think those things are worth trying although perhaps 7 is a little young.
And I do think that schools like to see the parents. It is a two way street and I think they like to see what you are like, are you going to be supportive and easy to work with. We have a younger son who is at a very snazzy prep school (but fab, the making of our older son!), one of his friends parents are constantly trying to take the school on, they dont agree with certain things, try and get things changed just for them.
This school is not for everyone, it prepares the boys for the big boarding schools and knows what it is doing, perhaps these parents slipped through the net and didnt meet the head..... They are a complete pain tbh, often writing emails to the other parents trying to drum up support for their latest issue.