You can choose starting characters and their children, but you can`t control what children will be born after the game starts. All those you set will appear in the family tree from the start. And because you have to have a certain amount of years between the parents and their children(17 years iirc) you can`t preset too many generations. All this is done in the descr_strat text file from Rome - Total War/data/world/maps/campaign/imperial_campaign. If you need more info on it you should check one of the tutorials in the
modding sub-forums of twc or on the twc wiki. Do a search there.