I always heard it said that a young cpl first becomes a 'family' when they have a child. The first child is such a 'Holy' and singular experience that can never be recreated. The three of them are a nice little family and then when the second and third born come along it is more of old hat. Despite the fact that Sonny later tried to mimmick his father's life work I think it took time for Vito to be willing to expose one of his children to 'that world'. His little 'sonny boy' was he and his wife's first experience in God's work and I always figured that he had a hard time letting Sonny see the mob world when he was young. Perhaps Mike was afraid that he had pampered his first born too much as well, and that it would take time for he as well to allow him to bring a son into 'that world'. Michael was the third child and Kay's aborted baby would have been the third child. He had screwed up by loving his first little boy too much and couldn't bring himself to let him in the business, but this was 10 yrs later and a third child(which would come during Michael's coldest time in life)would be just about the right time in his life to appoint and train a heir.

'This Sicilian Thing' of birthing little boys and watching them turn into powerful men who could help the 'family' rise in social strata is a tradition from ancient times that is of course not singular to Sicily, but many cultures that date to antiquity and before.

Luca wishes this upon Connie and Carlo so its obviously a habit and a tradition in their culture.