This au belongs to @the-officially-kat on Tumblr. The art is there's and I colored it.

"Patton!!" Dee and Thomas called as they ran through the house to get their nanny. They only stopped when they ran into their nanny's legs and fell down onto their butts. 

Patton yelped and turned around to face them. He rolled his eyes playfully and put his hands on his hips. "Were you trying to run me over?" 

"Yes!" The two chorused as Thomas giggled and Dee grinned like a goblin. 

Patton laughed. "You're gonna have to do better than that." 

The two children stood up as Thomas shouted a suggestion. "Ooo! Like with a car!"

Dee frowned. "No, like with a tank." 

Thomas stomped his foot. "No! A train!" 

"Boys!" Patton butt in. "Either of those would work. Now, what was it that you wanted?" 

"Can you join our tea party?" Thomas asked excitedly. 

"A tea party?" Patton started then paused to think. "Sounds great. Let's go!" 

"Yay!" Thomas exclaimed and the three of them began to journey to the playroom.

"So," Dee began. "I am a demon and Thomas is the King of Darkness, you can be a vampire bat." 

"Okay," Patton responded as they reached the boys’ playroom. Thomas ran over to the large chest in the corner and shoved it open. Dee followed and the two began to pull out the things for their costume. 

Thomas pulled on a black king’s jacket with gold shoulder epaulets and a blood-red sash.  Then he grabbed a midnight colored crown and rested it atop his head. Dee pulled on a gold-colored cape and red devil horns before he pulled out a pair of seafoam green bat wings and black bat ears. "Here," he said as he handed them to Patton, this is for you. 

"Thank you," Patton replied and pulled them on while Thomas set up the table, and Dee prepared the tea. He had to prepare two separate pots. One for him and Thomas with cyanide and one for Patton without the cyanide. Only those in the Addams Family could actually drink cyanide and not die. And Patton was the best nanny so they couldn't very well kill him and risk getting a horrible "normal" nanny. Dee set the teapots on the table and then grabbed his headless doll and placed them in the seat in between Thomas and himself. 

The three sat down. "We can now commence the tea party," Thomas said. Patton poured himself a cup of tea as Dee did the same. Patton made sure to keep his teapot close to himself so as to not get his mixed up with theirs. Thomas poured a cup for Dee's doll and slid it over to the doll as Dee's snake slithered over and wrapped themselves over Dee's shoulder. "Hello," Dee greeted his pet. 

Patton smiled and lifted his teacup and took a polite sip as Thomas looked at the teapots confused. They looked quite similar and he wanted to make sure he was grabbing the right one. "Dee, which teapot had the cyanide in it?" 

Dee paused and brought his teacup down from its previous path to his mouth. He frowned and glanced into his cup. "I can't remember…." He trailed off. Patton did a spit take as he had been in the middle of taking his second sip of tea. 

Dee and Thomas jumped. They frowned for a moment before it hit them. "Oh!" They exclaimed. 

Dee quickly reached forward and opened the teapot he had given Patton. He took a sip before turning to the other teapot and taking a sip. Within seconds, Dee had stood up so suddenly that his chair was knocked back. Thomas let out a shout as Dee raced out of the room to go find his parents. Thomas blinked before quickly following. Patton let out a yelp and followed after them, nervous that something was wrong with them. 

"Papa!!" Dee yelled as he raced through the house. He quickly found him and his Dad in the garden room.  His Dad was tending to his numerous plants while his Papa was painting a new train model.

However, with his loud entrance into the room, the two had quickly turned to see what had caused their son to make such a racket. "Dee?" His Dad, Virgil, questioned. "What's wrong?" 

"I poisoned Patton!" 

Roman and Virgil blinked. "Come again?" Roman asked. 

Dee huffed as Thomas and Patton caught up to him. "I poison Patton. Accident." 

"Well, it shouldn't be too bad," Roman chuckled. "What poison was it? I'm sure Uncle Logan can whip up an antidote." 

"Cyanide. I mixed up our teapots when we were having our tea party." 

Virgil dropped his scissors. "Oh." He blinked and looked to Patton who seemed to be trying to keep the panicked look off of his face. 

Roman bit his lip. "It appears we will have to hire a new nanny," he commented. 

"I'm sorry, I should have been more careful," Patton piped in. 

"Oh, it's not your fault Patton," Virgil responded. 

"No!" Thomas yelled. "I don't want a new nanny!" 

"Yeah!" Dee agreed. 

"What's this about a new nanny?" Logan asked as he entered the room. 

Thomas latched onto his uncle's leg. "Dee accidentally poisoned Patton with cyanide and now Papa and Dad are trying to get rid of Patton. 

"Oh, Thomas," Roman started as he stood up and moved over to his children. "You see-" he started as he kneeled down to be eye level with Thomas. "Only Addams can drink cyanide. Everyone else dies. That's why you'll have to get a new nanny." 

"What??!" Thomas exclaimed. "That's not fair!!!" He then turned and glared at Dee. 

Logan regarded the children before turning to Patton. "How long ago would you say you took the first bit of Cyanide?" 

Patton blinked. "Um," he rubbed the back of his neck. "A few minutes ago?" 

"Could you please be more specific."

"Maybe 10 or 15 minutes ago." 

Virgil, Roman, and Logan all froze in puzzlement. 

"A-are you sure, Patton?" Virgil asked slowly. The children watched the adults in the room with curious faces. 

"Yeah, it was a few minutes before Dee checked the teapots and ran here." 

The Addams adults stood still a few moments more before it began to sink in. "Ohhh," Roman breathed. 

Dee looked between Patton and his Papa. "What?" He asked impatiently. 

"Patton," Logan started. "Do you know how long it takes for someone to die from cyanide poisoning?"

Patton looked between the Addams men. "Uhhh, no?"

"5-10 minutes." 

Time seemed to freeze as Patton processed the information. "Uhhhhhhh," 

"But how!" Thomas asked loudly. 

"I thought you said only Addams's could drink cyanide," Dee said. 

"Right," Virgil answered.

"But-what-how?" Patton sputtered. 

Virgil walked over to Roman and slipped his hand around his husband's arm.  "Patton," he began. "Welcome to the Addams Family."