Normally, these three would be playing outside, but it's raining. So not to dampen anyone's spirits, Hiei decided to play with his 3-year-old son, Hiro and his niece, Yuki, who's also 3, inside. Today, they are playing Super Heroes. The game begins with Yuki running into another room and being held "prisoner" in the room by the kids' stuffed animal bad guys. Then, Hiro gets to fly, which means his dad will hold him up around his waist and run with him, giving Hiro the sensation of flying. Then Hiro finds Yuki, battles with the toys, and rescues her. Then they do this again.
The real reason is to tucker the kids out so they wouldn't object to a nap. Of course, this sort of playing is allowing Hiei himself to live sort of as a kid himself, through their eyes. Wasn't able to be a kid like this that long ago, so now playing games with these two at least makes him feel young.