......when he/she asks for a sibling?
My son started asking for a brother or sister when he was about 3. It went along the lines of: "when I have a brother, I shall do.......". "will I have a brother or a sister when I am 4?" When my brother or sister comes out of your tummy mummy..". "when I am 4 I want a baby brother"...etc etc.
I've tried explaining that not all children have brothers or sisters. He asks "why?". He says he wants a brother or sister to play with........
We won't be having any more children, and apart from now starting to feel my heart strings being tugged (and guilty) :(, I'm not sure how best to explain it to him :(.