At 12, Mycroft has much better things to do on a Saturday afternoon than follow Mummy and Sherlock around, particularly as Sherlock is insisting on ignoring the exhibits in favour of detailing all the ways someone could break in and steal them. He is doing this very loudly, and Mycroft is pondering calling Security himself. Sherlock shouldn't get an exemption from following reasonable behaviour just because he's Mycroft's brother.
"Look, Sherlock, dinosaurs," Mycroft says, because Mummy always likes it better when he plays nice, and most children Sherlock's age are obsessed with dinosaurs. Mycroft hadn't been, but he's always been an extraordinary child.
"Dinosaur bones are boring," Sherlock says. "Also, that one's fake."
Mycroft glances briefly at the dinosaur skeleton in question. "No it's not," he says, with no small amount of glee, because Sherlock is wrong. "Look at where those two bones lead into that joint."
"It doesn't matter," Sherlock manages while pouting outrageously. "They're still boring."
Mummy glances between the two of them patiently. "Mycroft, dear, would you mind looking after Sherlock while I go to the toilet?"
Mycroft fakes a bright smile for her. "Of course."
"Let's go over here, Mycroft," Sherlock calls out, immediately slipping away to head off in a random direction.
"No, Sherlock. Sherlock!" Mycroft calls after, with no response. Fine. Sherlock can run off if he wants, Mycroft isn't going to chase around him. Not when there's the list of donors to the museum sitting right over there to consider.
Mummy returns and barely bothers to show Mycroft how disappointed she is in him, but he feels it all too well. But really, it's not his fault Sherlock can't help but run off. If Mycroft had followed, Mummy would now be missing both her children. At least she still had him this way.
"Insects, you think?" she asks, and even if Mycroft knows that this is just her giving him the chance to make it up to her he still agrees and volunteers to go running ahead of her to check.
Sherlock isn't anywhere in the insect section, and Mycroft can see no indication that he ever was. He's also not anywhere near the primates, minerals or human biology sections.
Eventually Mycroft, with Mummy on his heels, finds Sherlock standing in the bird section, looking more frightened than Mycroft has seen him since Mummy explained that she and their father were separating.
There's a small crowd of people around him, offering help, but Sherlock is huddled in the middle, not responding to any of them. Suddenly he catches sight of Mycroft and goes running toward him, clinging to his leg.
Mycroft can't bring himself to resent Sherlock for it, choosing instead to awkwardly pat his younger brother on the head. Sherlock has never been big on physical affection, which usually suits Mycroft just fine.
"What happened?" Mycroft asks when the crowd has dispersed and Mummy has given Mycroft a glare and extracted a promise that he will make this better. She had then departed to talk to one of the museum curators for research for her next paper. Mycroft knows talking Sherlock out of his scare is the only way to earn her forgiveness.
"I got lost," Sherlock replies.
"Why didn't you ask any of the people for help, or go to Security?" Mycroft asks, because honestly, the odds of this happening again are rather high.
"I'm not supposed to talk to strangers," Sherlock says.
Every possibility of action and their probable outcomes flash through Mycroft's head before he decides to just drop to one knee and hug Sherlock. Instead of stiffening, as Mycroft had honestly expected, Sherlock relaxes into the hug and clings. "I hate birds," he mutters into Mycroft's shoulder.
"Of course you do," Mycroft murmurs back. "Want to go see the insect exhibit?"