It ended up being a ficlet, so I’ll just post it here:
John smiled at Sherlock as they walked back into Baker Street. It had taken some convincing for Sherlock to just go on a date, but well worth it. His husband had hardly seen the point; after all they were together every day anyway. But this was dinner and not a case and John was secretly hoping Mrs. Hudson wouldn’t mind watching Hamish for a little bit longer. He leaned up and kissed Sherlock as he pushed open the door.
“Pappa! Daddy!” John abruptly broke the kiss as Hamish crashed into them. John swung his head around and saw the incongruous sight of Mycroft sitting on the floor in front of the couch laughing.
“What the hell are you doing here?” John turned as Hamish tried to climb him. He picked up his son and glared at the elder Holmes.
Mycroft smiled. John held Hamish a little closer. “Can’t I come see my nephew?” Hamish squirmed down and went for the umbrella.
Sherlock put a hand on John’s shoulder. “You waited until we were gone.”
Sighing, Mycroft watched Hamish play with his umbrella. The boy was four and curious. “I have not seen much of him.”
John took a breath. “We’re not looking to keep you from him.” He moved to go make tea. “You’re welcome to babysit next time.”
Hamish got the umbrella open and crawled under it, peeking out at the adults. Sherlock sat down next to his big brother. “You are good with children.”
“I do believe John had other plans for when you got home. I could take Hamish out.”
A small smile crossed Sherlock’s face. “I believe Hamish would enjoy spending time with his Uncle.”
John reappeared with tea and handed one to Mycroft. Hamish came out from under the umbrella, something in his fist. “Uncle?” he asked.
“Oh, let me have that Hamish.” Mycroft quickly pried it from his hand before John could see what it was. “Probably you shouldn’t play with that umbrella.” He tugged it closed and put it behind himself.
John shook his head. Better not to ask.
“Mycroft is going to take Hamish for the night,” said Sherlock.
“Oh?” John raised an eyebrow.
“Yay!” Hamish climbed into his Uncle’s lap.
“Well, can’t argue with that.”
Mycroft stood up, taking the umbrella in one hand and Hamish’s hand in the other. John put a coat on his son. “You be good for Uncle Mycroft.”
“Yes Papa.”
They watched as Mycroft walked him out. John looped an arm around Sherlock’s waist. “I suppose he’ll be fine. Come on to bed.”