@Karwomannghia Have you sen this YouTube video of a mock linguistics interview at Oxford:
Jesus College, Oxford has put a lot of effort into helpful videos
What I did to support my DC doing ML interviews last year was to google lots / watch various YouTube videos from students and put together a bank of questions and they used those to prepare.
If you know someone who is prepared to do a mock interview using this bank then that is great. Linguistics is quite niche so don't worry about finding a linguistics expert but someone who is used to interviewing would be fine and you provide the questions. The school may be able to offer a Governor or you may be able to ask a friend or colleague.
It will help your DC simply to be asked questions and have to vocalise their responses out loud (hope that makes sense!). All about going into the interview a little more relaxed so they can show their potential.
Below is some useful advice I scrapped off the internet last year - I think it may well have been from a Mumsnet Post.
Oxford Interviews
As some who was (but is no longer) an interviewer at Oxford, some perspectives:
1. Lots of people apply and the interviewers do their very best to make the best decisions.
2. However, apart from about 10% of the candidate who are clearly not actually able enough, and the 10% who are out and out geniuses, it is extremely hard to place every candidate in the remaining 80% in the perfect order. We do (did in my case) our very best.
3. The goal of the interview is not to find out how much the candidate knows. The goal of the interview is to determine academic potential and things like ability to learn (on the fly), ability to quickly grasp ideas that are new, ability to respond to discovering that you might have been initially wrong and so on.
4. Another thing that is being evaluated is how well the candidate will gain from the Oxford approach to learning: big lecture, self study, intense individualised pressured tutorials. It is, in effect, a mini-tutorial.
5. Therefore, it is true that if someone doesn't grasp the questions well, then the questions will get easier. This can mean that the interview seems to go well for the interviewee, but is a non-offer situation.
6. Every year, a reasonable number of candidate fail to get into Oxford and Cambridge, not because they are not good enough, but because there aren't enough places for those who are good enough, but aren't absolute geniuses. Scant consolation I realise.