DYNASTY WARRIORS 9 Empires

If you finished one scenario already whilst having a spouse you should have a child officer available in the custom officers section, when selecting a character in your new game press LB to get to 'group 01' which shows the custom officers and then select the child officer.

Once in the game, have them relocate to the territory that their parent serves or rules and then choose to 'serve' which will net you the achievement for being in the same army as their parent, from here play out the game however you want until you eventually unify China and complete the scenario earning this achievement also.