I can't tell you of a specific site but there are some general things that an interviewer is looking for:
Personality - shy, confident etc
Personal interests - how sparky are they when discussing an interest and how much knowledge do they have of that interest
Lateral thinking - if asked a question such as what would you do with a million pounds
and they are really trying to see if a child has been coached either in interview techniques or academic tutoring. So if you're going to coach then coach them to pretend they are thinking of an answer. I have a friend whose child went to an interview and they asked her why she had come to the school to be interviewed. She said it was"preparation for another school"
, needless to say she didn't get in!
Personally I'm of the belief that a school should see a child warts and all and if they don't want them then the school didn't suit the child. I don't coach for exams and teachers with years of experience will be able to tell in the first 2 minutes of talking to a child if they are going to do well socially at their school. I also think a child can get stressed or nervous trying to remember everything that they have been told to say or how to act in an interview and therefore won't be relaxed and at their best.