Princess Kiko give a birth to ”the boy”

Under the current law, he will be Emperor in the future. At first, I think why emperor should be male, however, next question hit me soon;"Why should emperor be the first child?"
There are some opinion as the first child is smartest among his/her brothers/sisters, but it's not proved logically as far as I know. So, in my opinion, anyone whom the current emperor decided to be the next emperor could be the next emperor, even if he/she is not the first child and is not a boy. It's none of our business. Don't you think so?