This story is inspired by Dreadful-Kharma's fanfiction Bringing Up Kakuzu, which was discontinued and deleted. I hope you don't mind if I use some elements from your chapters...
Naruto Shippūden characters belong to Masashi Kishimoto.
Deidara was sure he heard a squeal. No, a lot of squeals... from Tobi's room. He decided to knock at the door. "Tobi? Are you alright, un?"
Another squeal and some laughter... Chipmunk-like giggles... Sounded like a baby. The bomber entered the room; the door wasn't locked, anyway.
"Tobi, un! What's going on?" Then he stared at an embarrassed Tobi... with a baby in his arms. "Eh?"
"S-Senpai, Tobi can explain..." Tobi stuttered, bouncing the infant. Deidara took a good look at the bundle: the baby had a tanned skin, a big mop of dark hair and stitches gracing his cheeks. His big eyes had green orbs and red sclera, just like Kakuzu's eyes. In fact, the infant looked a lot like a miniature of Kakuzu.
"Let me see him, un..." Deidara motioned to take the baby from Tobi's arms, but the masked man kept holding the bundle tight.
"Deidara-senpai, don't blast him, please!" He said, anime tears sprinkling from the hole in his mask.
"Tobi, I won't do that! Now let me see the kid, un..." Reluctantly, Tobi handed the baby to Deidara. "Hello, little one, un... What's your name?"
"Senpai, Tobi thinks he can't tell you that, because he still can't talk...", Tobi said. Deidara rolled his eyes.
"I know that, you idiot, I'm just breaking the ice, babies need attention, un!" And then turned to the baby, smiling. "Right, baby?"
The chubby little baby gurgled happily and spat bubbles, contentedly cradled in Deidara's arms. Suddenly, he startled at the sound of Hidan's yells in the corridors.
"OI! KAKUZU! WHERE THE FUCK ARE YOU?" Deidara sighed and looked at the baby with concern.
"Don't mind Hidan, little one, un... He's, um, kind of nice, once you get to know him..."
"Oh, senpai, about that... Um... You see, this baby is..." Tobi was interrupted by Hidan, who stomped into the room like a demon.
"Haven't any of you bitches seen that asshole around?" Hidan stared at the baby in Deidara's arms. "Why, Blondie, I didn't know you had a fucking kid... Jashin, is he weird..."
The blonde sighed, annoyed. "Very funny, Hidan, un... It's not my kid. Tobi found him."
"That's what Tobi was going to say, senpai!" The masked man raised his hand. "Tobi didn't find him! The baby is Kakuzu-san!"
Hidan and Deidara exchanged funny looks and started to laugh, as if they'd just heard a good joke. The baby, sucking on a thumb, looked at Hidan curiously.
"Tobi was practicing an age-reversing jutsu, and Kakuzu-san asked Tobi to perform the jutsu on him, so he could be back to his twenties, and Tobi happily did and... Well, the jutsu worked, but Tobi doesn't think he's in his twenties right now..." Tobi said, scratching the masked head. Deidara and Hidan were gobsmacked.
"Well, can't you undo the fucking jutsu? I'm not gonna take care of that little shit.", said the Jashinist, pointing at the innocent baby that was playing with Deidara's hair.
"Tobi can't, Hidan-san! Tobi tried, but nothing happened.", squeaked the good boy, hiding behind his senpai.
"What's all that noise?" Sasori peeked through the door. "What the hell is that?" He entered the room, cautiously.
"Danna, this is Kakuzu... That idiot Tobi has performed an age-reversing jutsu on him and he's turned into this, un..." Sasori took the gurgling baby from the blonde's arms, so he could have a glance at him.
"Kakuzu... You're tiny..." He gave a little smile to the stitched baby, earning a cute baby giggle. Sasori turned to Hidan. "You're his partner, Hidan, now it's time for you to take care of him..."
"WHAT?" That made little Kakuzu startle again. "Why would I take care of that fucking bundle of misery? It's Tobi's fault, not mine."
"But Kakuzu always took care of your wounds, Hidan, un..." Deidara had a point. But that wouldn't convince Hidan. "By the way, Tobi, does Leader know about that?"
"Not yet, senpai! Tobi was scared of what could happen to little Kaku-chan..." Tobi's nickname for the baby made Hidan cringe.
"Well, then, let's show him to Pein, what could possibly happen?", said Sasori, rubbing the baby's back lightly.
"Show me what?" Pein entered the room, stoicly as always, followed by Konan. He glanced at the bundle and Tobi whimpered.
"L-Leader-sama, it's Tobi's fault, Kakuzu-san wanted Tobi to perform an age-reversing jutsu on him to make him younger, but... He's got too young..."
Sasori handed the baby to Pein. "We don't know how the stitches retained in his baby form... But it's Kakuzu, really."
Awkwardly, Pein held the baby up at eye-level, examining his features carefully, while Konan smiled softly at him. The baby grew a little bit tired of being examined and started to pout and blush, he was about to cry his big eyes out. Konan took him from Pein's hands and shushed him, patting on his back gently.
"It looks like we're going to take care of him until the jutsu wears off.", said Pein, pinching the bridge of his nose. "That, or we can simply abandon him in the forest..."
Everyone was about to protest, but Hidan was the first, yelling, as always. "ARE YOU OUT OF YOUR FUCKING MIND? I WON'T LET YOU DO THAT!" Once again, the yell startled baby Kakuzu. Pein smiled smugly.
"That's what I wanted to hear from you, Hidan." Then he patted the baby's mop of hair and smirked. "You'll be alright, Kakuzu..."
Konan handed Kakuzu to Deidara. "Can you get him something to eat? I'm going to make a list of the things the baby will need. As soon as I finish it, Tobi, you're coming with me to go shopping, okay?"
"Yatta! Tobi's going to help you, Konan-san! But... We're going to need... You know..." He whispered. "Money..."
"That's easy, you can use the miser's fucking money, I'm sure he won't mind... And the little shit's not in any position to complain, is he?" Hidan smirked at the baby. Kakuzu understood the word "money" (he was Kakuzu, after all) and the lip-quivering started. "And don't fucking cry, dumbass!"
"Waaaaaaaaaaaaaaaaaaaaah!" Too late; Kakuzu wailed at the top of his lungs, rivulets of tears running down his sad face.
"That was mean, Hidan!" Deidara put the baby on his shoulder and patted on his back lightly. "There, there, little Kakuzu, un... Uncle Dei's going to fix you something to eat." He went to the kitchen, followed by Tobi.
"Senpai! Tobi made some onigiri, maybe Kaku-chan likes it! Let Tobi help you!"
"So you really care about your partner, don't you, Hidan?" Sasori said, laughing a little.
"Fuck you! I don't give a shit; besides, we're gonna need him when the fucking jutsu wears off." The Jashinist scowled and crossed his arms over his chest. "I don't care about his fucking upbringing..." Suddenly, he stomped off to the kitchen to yell at the good boy. "Oi, oi, Tobi! That rice ball is too fucking big for Kakuzu's mouth!"
"Sure you don't, Hidan." Sasori went back to the room he shared with Deidara, smirking.