It seems kind of strange that EA allows young adults to have teen children, but disallows adults from having young adult children... Why EA? Anyhow, I use XCAS as a core mod. . . and a'm not willing to give up its ability to edit the "nude" outfit in CAS (so Awesomemod is out of the question) and MasterController. Right now I am trying to make an adult (in CAS) with a young adult child. Anyone have any Idea how I can accomplish this?

Can you not just make the child a teen in CAS then age it up to YA once you put them in game?

Are you implying that you have MasterController or that you don't? Because there is an interaction with MasterController that allows you to add the young adult as the adult's child. Even if you don't have MasterController, it won't interfere with XCAS as long as you don't have the XCAS version with expanded tattoo locations.