A little bit of kid John and kid Sherlock fluff, inspired by Mycroft's line in A Scandal in Belgravia. "Initially, he wanted to be a pirate." Thanks to Suki-Chan36 for the word suggestion.
First mate John and Captain Sherlock settled into one of the huge oaks in the back garden, attempting to smother their laughter as Mycroft combed the grounds looking for them. John had shown up wearing one of his favourite striped jumpers, and the resemblance to a sailor's uniform was so uncanny, Sherlock couldn't resist dressing them both up.
They'd stolen a couple of Mummy's scarves to wear as headgear, and Sherlock had appropriated one of his school ties to use as a belt. The reason they were hiding was that Sherlock had insisted no pirate outfit was complete without a sword, and besides, Mycroft never even used his fencing foils anymore. They'd be much more useful to fight off crocodiles and naval officers, anyway.
They'd managed to evade him long enough to get out of the house, and they'd gained the upper hand by scaling the tree before he got outside.
John's shoulders shook with stifled mirth as Mycroft plodded across the yard, muddying the cuffs of his trousers. The undisguised glee on Sherlock's face proved too much though, and John let out one huge guffaw, leading to a cascade of giggles from both of them. That captured Mycroft's attention, and he was immediately at the base of the tree, yelling at them.
"Get down from there, you filthy little buccaneers!"