Thinking about Sukuna and Saiki meeting at children. Saiki's powers going slightly haywire while he's sleeping and getting to the heian era by accident, meeting a very scared Sukuna in front of him.
They somehow become friends, mainly cause both of them are lost and Saiki knows how to defend himself that Sukuna can't eat him like he usually does to people, even as a kid.
Sukuna, as a kid, is fascinated by Saiki's powers and is also confused on why Saiki isn't scared of him. Saiki is first to not try and kill him, just for looking how he is, four arms and four eyes and a mouth stomach.
Saiki, as a kid, finds Sukuna fascinating. Sure he's a little frightening, but four arms, four eyes and mouth stomach? How does that work? His anatomy looks off yet he's alive, even if he's fighting for his life in this forest. Kusuke would have a field day with Sukuna, though it's better for them not to get know each other. Saiki doesn't want Sukuna to become an experiment and more of a monster like he is.
Saiki does eventually go back, mainly by his powers putting him back into his present. He's a little disappointed when coming back, but is happy to see his mom. Sukuna on the other hand? Well, he doesn't take Saiki's absence to kindly.