It’s insecurity of parents and lack of familiarity which leads some parents to put their child in a suit, mistakenly imagining that such a school expects 10 year olds to dress in suits.
The purpose of the interview is prob marketing, confirmation of what exam has already suggested and adding a personal touch to the process. It’s about meeting the boys and getting a bit of a sense of them - that is achieved through what they say and how they are, not their clothes. Yes, schools want a ‘type’ and it’s ideally clever, enthusiastic and willing to join in. That can be a boy from a state or prep school and a boy in most types of clothes 10 year olds wear.
Honestly, top schools don’t judge on dress - as has been said, being over-dressed is often looked down on and is ridiculous in a small child. Top schools want clever, enthusiastic boys. Perhaps there are some 5th rate tiny private schools which achieve little and are there for snooty value who would judge by dress, but these aren’t the good schools.
Clean clothes, polished shoes. Uniform of any time a simple and easy approach - and sod the idea that a state school uniform leads to ridicule. Again, only in 5th rate snooty places. And look out for lots saying ‘come in what he feels comfortable in’ - they mean it! So no Mohican haircut or offensive slogans, but simple usual clothes are fine on that context and you’d be hard pressed to pick something ‘wrong’.
And now what to wear as a parent....and do you need a designer handbag and will your car be clocked. Ha ha!