I don't have specific experience of that wonderful choir, but I can give you some general pointers. First and foremost, they will be looking for potential, so don't worry if your DD isn't the finished article. They will be looking for a sparky and enthusiastic personality, so encourage your DD to emphasise this. Pitch matching is very important, so it's good that you're practising the aural tests- little and often will make a huge difference. Has she prepared a song and if so, what is it? A hymn is often a good opportunity to show a clear and pure sound.
A couple of points specific to the current situation: presumably it is a Zoom audition (or similar), so do lots of practice of singing/chatting on video calls (with grandparents/whoever)). If your DD is comfortable in this medium, it will set her apart from other children who may flounder. Also, Cathedral Choirs are struggling at the moment and it may be a bit easier to gain a place than usual.
If they interview you or have you involved in the call, show that you're 100% committed to your DD being involved next term in whichever way the choir requires, whether it's the usual mix of before and after school rehearsals, or Zoom rehearsals (if still necessary), or a mixture.
If you have any other questions, don't hesitate to ask- I work in this field. Good luck to your DD- I've got a couple of recordings by the St Alban's girls and they are sublime!