A hybrid of previous answers:
If the child is 14 or older, I believe Zyggy is right... they must appear at the interview.
If the child is under 14, you should probably contact the Consulate and ask. My husband (then fiance) hand-delivered Packet 3 to the Consulate (Asian post) and was told to come back THE NEXT DAY for his interview and to bring his son (9 y/o) with him. Our son was interviewed separately from my husband, and whereas my husband was interviewed in English, our son was interviewed in their native language. They asked him questions like "when is the last time you saw your "old" mommy?", "have you ever met your "new" mommy?", "did your "new" mommy ever send you anything in the mail?" and a few others that I can't remember. I don't know for certain, but I would think that responses of a young child are excellent evidence of a valid relationship (as well as potentially a double-edged sword). If I were a CO, I'd be very interested in hearing what a child has to say about the situation.
Best wishes for a successful interview!
Maya