I am not average person, you don't look to be one either.

Fair enough, but I still don't think I am very good at predicting whether I'll be happier with children. I also doubt that other people who do think they will be happier are very accurate. Humans are notoriously bad at determining what will make them happy/unhappy. I'm thinking in particular about the study about lottery winners vs. amputee victim from Dan Gilbert's TED talk: http://www.ted.com/talks/dan_gilbert_asks_why_are_we_happy.html.

if your idea of meaningful life is improving the world, I don't see how you can have a sense of meaning and at the same time be aware that you're not "doing reasonably well".

Society as a whole regards having children as profoundly selfless, rather than selfish, so I think I am fair in concluding that some of the sense of meaning that people get from having children is related to improving the world for future generations. That particular self-satisfaction might not be disturbed by Rachaels' argument if one does not take moral arguments seriously.