i think the mod you want is NRAAs "porter" it essentially lets you save your sim families back into the bin with all of their relationships intact (it works for multiple families/households as well). 

So you could create your world as you like it, save it. grow your sims up in whatever hood you like, then use porter to save them. load up your original saved file and use porter to import your sims to your new world. 

make sure you follow the readme, though. its a pretty straight-forward process, but i believe there are  couple of "you must do this, or all will be lost" steps.