Great thread! I can totally relate. I already have two (so thankfully they have each other) but they desperately want another brother or sister. And I mean DESPERATELY! They've been asking non-stop for over two years. The youngest (daughter, 6 years going on 7 soon) is just obsessed with babies and has been her whole life. She has had a lot of friends lately that have had baby brothers and sisters. She's so excited for them but in private asks me when she will get to be a big sister. It just hurts so much.
And my older son is the same way. He just loves entertaining children that are younger than him. Whenever there's a baby around, he'll play and talk to the baby until the baby gets bored. My children have entertained random babies and toddlers on public transport, in theme parks, at parties, etc. They would make such a good older brother and sister.
At first we didn't tell them we were trying but my son (who knows wayyyyy too much now) kept asking if we were doing "that thing" (his words) that makes babies.
We told them we did want another child and they would be the first to know when it happens. I look forward to the day when I can tell them that yes, they're having a new brother or sister. It will be a happy day...