Come prepared with answers to questions the interviewer might ask you. You don't want your answers to seem rehearsed, but you do want to sound ready. Possible questions include: Is your child independent? Can she work on projects on her own? Has she spent time away from you? Does she enjoy participating in group activities?
Research the preschool director's background. Come to the interview armed with a few tidbits about the work the director has done with the school so you can offer some praise during the interview. For example, if the preschool director created a sculpture class for the kids, mention that you find that impressive.
Rehearse with your child. Explain the upcoming event as simply as possible and practice showing him how to sit without fidgeting and how to answer simple questions. Practice questions such as "What's your name?" or "How old are you?"
Dress your child in a semi-formal outfit such as a button-down shirt, sweater vest and a skirt or slacks. Make sure your child is well rested and well fed; you don't want to bring a cranky, crabby child to the interview.