DYNASTY WARRIORS 9 Empires

Firstly you need to have a child. To do this start a conquest mode and build your relationship level with a officer of the opposite sex until you can "confess" to them and get married.

You then simply need to finish the campaign with your spouse alive (they can be killed if captured during a battle you lose there is a small chance the enemy will choose to execute them) to get a cut scene at the end with the birth of your child.

Save this "child" to a created officer slot and start a new campaign where your parents are in a army. For example I chose Ma Chao and completed the yellow turban campaign, had a child at the end then started the alliance against Dong Zhou campaign with the child as a vagrant/free officer. As soon as I was able I moved to Ma Tengs area and requested to join his army. As soon as he accepted the achievement unlocked.

It's worth planning ahead slightly or being familiar enough with the story to know which officers are going to be in armies from the start of a campaign for example having Sima Zhao as a parent because you used him on the yellow turbans is not going to start in a army on the Dong Zhou campaign as he doesn't come into the story until much later.