You want to get a sense of her experience with infants versus older children etc. Just hearing how she talks about the children will provide you valuable feedback. Does she sound positive, bitter, loving, indifferent?
Can you tell me about the families for whom you've worked?
Does she have positive things to say about her past employers? Are they currently on good terms? Why did that position end? What other duties has she been comfortable performing?
Invent something rare and extreme like say a house fire and something ordinary and routine like a child spilled something on the carpet. She needs to be prepared in the case of an emergency but she also needs to have the temperament to handle the everyday challenges of child rearing.
How might you spend the day with my child?
Can you tell me about an experience when you had a difficult time with a child or parent and what you did to resolve it.
How many days have you had off sick in the last 12 months?