If I played the game realistically, my Sims kids would move out and find a place of their own. I would probably have the parents help get them started, and they would visit, but they would be on their own.
However, I don't play the game realistically. My plan for my Sims kids would be that the heirs of each family would move back into their childhood home with their parents, and start a family. That way the house will stay with the family, and keep the family generational. Plus, they would have the most kids. As for the kids that aren't heirs, they would get some money to move out, and are free to do what they want. If they have kids, I will limit them to only one, so that the Sim population doesn't cull the family. So, overall, heirs return home and live with the parents, but their siblings that aren't heirs move out and do what they want.
