Firstly, she has probably done so well thus far because she has not been prepped. Please try not to prep her as any answers will come across monosolabic if she is trying to recall what you told her to say.
If you are asked why you have picked the school I would act pretty niave about the whole process. If they think they are getting a parent who doesn't know the ropes then that parent is less likely to complain about any issues - perfect in their eyes. Tell them you like their school because out of all the ones you've seen they really seem to nurture the children.
You may be asked what senior school you're interested in (if school finishes at 11 or 13+) and you can drop in that you are unsure but this is another reason you have chosen this school as they get kids into a wide range of schools which makes you feel confident that the school will offer the right advice later on. Might want to drop in that a Relative has kids are at a named Public School (I don't endorse fibbing, but...) and you are interested in keeping it in mind; leavers lists are important to schools and if half the job is done before a kid enters then that's a big bonus. The balance is to say the right things which pander to their ego without sounding a suck-up or desperate!
To be honest, you are on trial not DC - she has pretty much already passed! Good luck and let us know how it went.