First of all, I would ask them why they want to be an au pair, and perhaps what it is about my family that attracted them to the job. I would ask what they plan to do after their time as an au pair.
Then, I would tell them what the job is and see if they are in agreement. Emphasize the amount of housework they would have to do.
Then, I would ask what they like to do with the children - craft, cooking, playing outside, boardgames etc.
Then I would ask them what they will do during their time off - it's good if they have an interest that will occupy them.
Then I would ask them if they had any specific requirements - food, allergies, religious observence etc.
Personally, I look for an au pair that is going places. The time as an au pair is purely a gap year to take time off formal studies, to improve English and to get an idea of British culture. What I don't want is someone who is drifting through life. Basically, I want someone that is bright and full of initiative but is willing to get stuck in with anything.