depends on the family, sometimes I'll leave the kids I don't play with their parents and switch between the two and other times I have branched out, one of my oldest families I no longer play I think I lost them, I let the youngest keep the house and the first and second moved out and had their lives before resurrecting them to play the second's households and the child of the first. As one side was a vampire and the other wasn't. It's fun if you're in the mood for it you decide how different or how similar each one grows up like it's a competition for who has the best life, otherwise well whichever one I like the most.
