[Brat]

A 10 year old taunting you about your childlessness? Really? If it is indeed taunting, that's pretty bad manners.

Personally, if children show bad manners, I leave it up to their parents to deal with them. I know that if I were to taunt an adult about anything, much less infertility, I would be on restriction for life. If you really sense the child is purposely trying to hurt your feelings, I would tell the parents exactly what the child has said and the context, that it is intolerable, and that you would prefer that they speak to her. If you think, however, that her question is an innocent one, and that maybe it just feels like taunting because you're feeling particularly sensitive right now, still ask the parents to ask her not to ask that question anymore.

Beyond that, I wouldn't feel obligated to share the intimate details of conception with someone else's child, even if it were a niece. That again, is up to the parents. If they decide to discuss infertility, I would ask them not to share my personal, intimate details--especially knowing that child will likely share them w/others and probably with my future child.

With your niece, I would stick to telling her about the positive aspects of adoption, educating her ahead of time because once her cousin joins the family, s/he will benefit from being treated respectfully. The child who taunts you, will taunt your child, unless this is nipped in the bud.

As for your four-year-old niece, I would say that I don't know when I'll be having a child, but hopefully soon. If you're a person of faith, maybe say that only God knows. Then change the subject. I can't imagine asking that question at such a young age, and she's probably just following the lead of your other niece. At any rate, you don't need or deserve to walk through a minefield with children. Let their parents deal with it. If they don't, sadly, perhaps you will need to limit your interactions with their family.