I got this World War II AU idea from a staying with cousins prompt on a prompt list and, well, it turned into this:
Only now Mycroft was sending him away too. He’d told Sherlock London wasn’t safe, that he’d be fine with great-aunt Mabel, that he’d be home before he knew it. But Sherlock knew he wasn’t certain. The war wasn’t going very well so far and there were rumors even among his schoolmates that the Germans could be bombing London within months.
“When you get off the train Mr. Harris will be waiting for you,” said Mycroft. “Write me,”
For the record Sherlock is thirteen and he was in the care of Mycroft because their parents were busy with other things related to the war.