Author note: I do not own Naruto. Kakashi, Rin, and Obito are 12. Minato is 23 and isn't Hokage yet.
The team had just finished their mission. Minato dismissed them quicker than usual. As he was walking away, he pulled out a book. The three Chunin watched as he walked away. "Is Sensei really reading a book called Make-Out Paradise?" asked Obito. Kakashi and Rin nodded surprised. None of them thought of him as a pervert. "He wouldn't read a porn book in front of us, would he?" questioned Rin. Obito replied, "No, of course he wouldn't. I wonder what's in it?" Kakashi and Rin were just as curious. They decided that they would follow him the next day.
The next day the three Chunin met, exactly at the time they planned. They were definitely serious now. They were in disguises. Rin wore a blond wig and a poofy pink dress. Obito didn't wear his goggles or family crest. He also had a blond wig. Kakashi didn't wear his mask. He also had a blond wig. They found Minato and started subtly following him. They watched as he walked into the grocery store. "Kushina lets him do the shopping? They must eat a lot of ramen." whispered Obito. Rin and Kakashi agreed. They watched as Minato and Jiraiya ran into each other. They heard them laugh as Minato reply to whatever Jiraiya had said. Unfortunately, they didn't hear what was said. Jiraiya had asked about how long he had been getting followed and Minato knew exactly how long. But the students didn't know. They continued following their teacher, until he went into the ANBU lounge. Only current or former ANBU agents were allowed. They heard the punishment for entering was so horrible that it couldn't be spoken.
Finally Rin suggested, "We could just buy a copy and find out that way." It was the best idea they had, so they went to the bookstore. They found it quickly. They bought it up to the cashier. "You can't buy that. You children are underage." Their faces had a look of complete disgust on them. Their teacher read a porn book in front of them. Minato was walking by as they were leaving the bookstore. "Hey! I have a mission for us." said Minato when he saw them, "Why are you guys dressed like that? Are you trying to act like you really are my kids?" The last part had been a joke, but they had all became even more flustered.
The mission was a simple one, harvest the farmer's crops for him. The Chunin were the only ones that worked, the Jonin read "the book". By the time they finished, Minato had fallen asleep with the book in his hands. "He won't know if we take a quick look at his book. If he wakes up we can tell him he had dropped it and that we were picking it up." said Kakashi The children smiled evilly; it was a perfect plan. They approached him carefully. They all knew he wasn't a morning person. Kakashi carefully grabbed the book and held it between them. They looked at the page their Sensei left off at. At first they stared at in silence, trying to figure out what it meant. When they figured out what it meant, Minato woke up to Rin screaming and Kakashi and Obito trying to get the book away from them. Minato just stared at them drowsily, while his students stared at him in disgust.
Minato later figured out what they had done, but by then it was too late to confront them about it. Kakashi, Obito, and Rin later started reading those books as a pastime. They used the transformation and substitution jutsu to get the books. Jiraiya had turned Minato into a pervert and Minato made all his students perverts.