The children print in the order of the sort birth date. If the sort date is blank, they print before the ones without a blank sort date, I think in the order they were entered.
You can see the sort date by clicking on the event number next to the birth event. There will be three dates, as entered, formated and sort. They do not have to be the same, so if you want to enter a sort date in order to get the children in the correct order, it is normally not used for anything else. (There is a bug right now that gives you a false age at death if the sort date is entered)