I think that Sukuna is p reluctant about having a kid, even after Yuuji gives birth. But then one day their kid gets scared by something (a storm? A bug? A person?) and who do they immediately seek out? Sukuna. Bc their dad is the strongest and he has extra arms to hold them and keep them safe. And then it just clicks for Sukuna, like a pet dad letting the new kitten sleep on him for the first time