Hi
Our DC sat for 3 schools, all of which were genuine contenders, but one of which was easier to get into as the others were quite academic and sought after. Any more and there's a danger of burnout. The worst case would be that the preferred school is the final exam, when they are exhausted and bored, after a string of them. As you say - they're only 10 at the time!
In addition, as prep, they did a few grammar school mocks - Sutton organises some and there are a few companies who do similar. they charge, but not a lot. These exams weren't identical in format to the independent exams, and the DC knew it, so they didn't expect to do especially well in them and we didn't even bother telling them the results. It was training for getting used to exam conditions, finishing papers on time and shutting up and sitting still for up to three hours at a stretch, which no state primary school child I've ever met has practise in!
I agree, practise the interview a bit, but don't do what I did and start asking questions as though you were the interviewer. That idea was ditched after one question. My, how that bombed! Just ask what they might answer if asked xyz, and then, if you think they are way off beam, chat to them a bit.
DC were asked, among other things:
Tell me a joke
If you had to spend a million pounds, but not on yourself, who or what would you spend it on and why?
Where did you last go on holiday?
They were also asked some mental maths questions. Square roots or primes - that sort of thing.
And the killers:
What is your favourite school from the ones you are applying to?
What is your parents' favourite school?
Those two questions could throw a child if he or she hasn't thought a bit about the answer in advance.