The world of Sherlock Holmes was surely full of imaginary friends.
Young boys of the Baskerville family and their curse dogs. Garrideb cousins coming over to play with little Nathan. Pre-teen Mary Sutherland's beaus who travelled exotic foreign lands like France.
While we don't get a lot of children in Watson's case records, observed at play when the adults are not around, it's not hard to see from the adults in those tales that many an imagined playmate must have existed among that populace.
Little Jacky Ferguson and his vampire pal, who'd come to deal with bullies and other threats to his happy domestic life, like an immortal neighbor girl from Let The Right One In.
Little Lucy Hebron and her bright yellow sister, who would look out the window for her and see what the weather was doing.
Little "Smacky" Rucastle and his giant, who'd crush local townsfolk under his boots just like Smacky did with the cockroaches of Copper Beeches.
There is so much going on with the citizens of Canon Holmes that the lives of their offspring could not have been any less imaginative.
John Watson must have had some experience with the wilder of their breed, else he wouldn't have been so quick with the pronouncement, "Holmes, a child has done the horrid thing," after a murder in The Sign of Four. Perhaps he had crossed paths with a Moriarty offspring at some point without realizing it . . . and just think of the imaginary friends that Moriartys would play with. Or don't, as a little Jimmy Moriarty sort of kid problem had a complex organization of imaginary friends all performing chess-move-like imaginary stratagems. Hard for the average mind to fully envision.
Did Sherlock have an imaginary friend that also was on his way to becoming a detective? I can't help but return to "Barker" from "Retired Colourman," with that supposed rival being the opposite of BBC Sherlock's Redbeard -- a dog that Sherlock pretended was human. "Barker" -- get it? But other people did see Barker, I suppose, so I guess he wasn't imaginary . . . unless that was a random stranger Holmes just said was Barker.
Ah, well. I should not watch SyFy channel's Happy! before bed . . . too many Sherlockian twistings of a sleepy mind!