I have a legacy of 15 generations, and this is my second legacy. The other one only had 7 generations I think but that's unimportant. I always have lots of kids because I love making huge family trees on familyecho.com and also because I love toddlers in The Sims 4.
When I choose the heir to move on to I usually go with the prettiest one... Or just the one I like the most for whatever reason. It really doesn't matter. With the others I usually move them out and then give them a spouse via the "edit household" button in manage world view. Sometimes I give them one or two kids. I do have MCCC commander that lets the sims you don't play have families on their own but I do tend to create spouses and children manually. Sometimes I choose to leave them with the old parents and pretend they're taking care of them.
If I love the current generation's parents then I make them have less kids so that he household will be smaller and they'll have a closer relationship. Then I have the heir stay with the parents until they die of old age to, well, make the bonds and family ties more realistic. Creating a story, kind of. What makes TS4 especially fun for me is having a kid and then imagining what they're gonna be like when they grow up and what their own kids will be like. What kind of new houses I'll build for them. Always moving forward.
One thing I do is have one child of the parents in every generation take a picture of the mom and dad. Just one picture where they stand next to each other and look into the camera, and then save it as the biggest format and frame it. I do this with every new generation so there's one picture of every parent dating back to the beginning. It's just a fun thing to do and look at when you line them all up in order on a long, long wall.
