I like this quote from current interview about them:
The couple have two adopted children (two judges investigated each adoption, as is routinely done, and okayed them) because of Soon-Yi’s strong convictions about the narcissism inherent in having biological children. “I could definitely have children,” she says, “but I was never interested. I find it the height of vanity and very egocentric. I don’t need kids out there who have similar traits to me and look similar to me and Woody. Why is one’s DNA so special? Why would one keep on breeding when there are so many kids out there who need a loving home?”
http://www.vulture.com/2018/09/soon-yi-previn-speaks.html