Suho, or Junmyeon was an adult who loved children. His personality was what made many girls fall for him (other than his face) His carefree, selfless action sometimes just amazes elders and even children. (He's the owner of the place, and owns the whole building, where kids can take naps and is basically a shelter for kids who was abandoned or their parents have no time) 

He was the person who often helped children learn how to swim, although he was the type of kid who never would really tell them the actual moves like the butterfly and stuff like that. 

Kris, or Yifan was the same and worked in the same place as Suho. Although, he was the total opposite of Suho and is just the more serious type. He has a cold personalty, or so the others see it. 

Although he seems that way, many children also come to him because he does the same with the kids, just encouraging them and helping them to swim without teaching them the proper ways. 

The other EXO members.... they're kids in this story. They're the kids who takes the lesson in the place which Suho owns. (Kris is... the late assitant..)