Sherlock Holmes Costumes
You stand straighter with the coat collar turned up, eyes moving first, cataloguing the scuffed shoe, the inked sleeve, the room tidied to hide something, and when someone asks how you knew, you say only "elementary" and let silence finish the thought. People spot you before you speak: the long coat, the unlit pipe, the double brimmed hat, a child tugging a sleeve to point you out, an adult nodding and already playing along as your Watson or your suspect. That instant recognition is the fun of it, everyone already knows the character, so the costume only has to nudge them the rest of the way.
The classic detective coat
This is the version most people picture first: a long coat with a full silhouette, a pipe as a prop, and the deerstalker hat that instantly reads as "detective" from across a room. It suits an adult who wants to commit fully, someone who enjoys holding a pose and delivering a line rather than just wearing an outfit. It works equally well for a quiet, observant type who likes the idea of being the smartest person in the room without having to say much, and for someone more theatrical who wants an excuse to narrate the evening out loud.
Sized to fit properly
A costume that pulls at the shoulders or won't close at the front stops working the moment someone tries to move in it, and a coat is most of this character's effect. A version built for a fuller frame keeps the same coat, hat and pipe details but is cut so the wearer can actually sit down, raise an arm to point dramatically, or lean over a magnifying glass without anything straining. This matters more for this costume than most, since so much of playing Sherlock is physical: the sweeping walk, the sudden turn, the coat settling back into place.
The young detective
For children, the same idea comes scaled down and simplified: a coat, the recognisable hat, and enough of the silhouette that any adult who sees it immediately understands the reference. This suits a child who likes solving things, who asks a lot of "but why" questions, or who has been narrating their own adventures around the house all week. It also suits a shyer child well, because the character does not require loud energy. Sherlock observes quietly and speaks only when he already knows the answer, which is a comfortable role for a child who would rather watch than perform.
Who it suits
Sherlock Holmes works best as one half of a pair. A family or a couple can split Holmes and Watson without anyone needing a third accessory, and a group can add a suspect, an inspector, or a client with a problem, since the whole story only needs two people talking and everyone else reacting. It also stands alone perfectly well: a single detective walking through a party in character is its own small performance, no partner required. For a shy child, it is one of the gentler characters to try, since it rewards watching over showing off.
Playing the part
Stand still for a beat before you speak, as if you are finishing a thought. Walk with your hands clasped behind your back or one tucked into a coat pocket, head tilted very slightly, eyes scanning the room like it's a crime scene. Pick a single catchphrase and use it sparingly so it lands each time: "the game is afoot" as you enter a room, or "elementary" delivered flatly, without a smile. For a party game, hide a small object and let the children take turns finding "clues" (a dropped button, a footprint drawn in chalk, a note left on a table) and reasoning out loud about what happened, the way Holmes would.
A detective's hat pulls the whole performance together even on its own, if a full coat feels like too much for the occasion. Put it on and the posture tends to follow.
Finishing touches
The hat is doing most of the work here: its double brim is the single most recognisable shape in the whole character, more so than the coat itself, so it is worth getting right even if everything else is kept simple. A pipe held rather than smoked adds a natural place to put your hands and a reason to pause mid-sentence, as if you are thinking. Keep everything else understated: dark, plain colours let the coat and hat do the talking, and a magnifying glass borrowed from a drawer at home rounds out the look without needing anything else added.