We went through this last year. First, I would speak to the registrar and ask what sort of child does well at the school. Then ask her to tell you a bit about the interview and what to expect. She may give you some usable hints. I would also ask about practical things such as: how long is the interview? will your dd be interviwed with other children or on her own? Will you be allowed to stay with her? Speak to your dd about these things in advance so she knows what to expect.
On the day of the interview, I would make sure to arrive in plenty of time so that it is as stressfree as possible.
In my experience, they are looking generally for a child who is eager to learn, who can join in with other children in a group setting, etc. We went through several interviews. They varied, but were fairly similar: asked my dd to write her name - mainly I think to assess her pencil control. She was asked to identify some written letters/numbers, did some matching games, was read a story and then asked a few simple questions to see if she'd paid attention/retained anything, asked to identify some basic colours/shapes, shown a picture and asked to count how many balls/stars there were etc.
Do you know other parents who already have children at the school? Could they do some informal lobbying on your behalf - perhaps in the form of a letter. Don't know if it helps, but it couldn't hurt I don't think.
It is a nervewracking time. If I were you, I'd think about a backup plan in case she doesn't get an offer. By being prepared, you'll celebrate if she gets in, and won't completely panic if you're obliged to look elsewhere. Good luck.