Think of the children in opera. Not knowing sopranos and mezzos, pigtailed and pinafored or tightly trousered-up to look child-like, but actual children. There are Mozart’s Three Boys, Menotti’s Amahl, possibly Debussy’s Yniold and Handel’s Oberto and, if you stretch a point, Marie’s little son in Wozzeck. But that’s about it. Until, that is, you come to Benjamin Britten.

It’s a rare Britten opera that doesn’t include a child. Whether it’s Grimes’s doomed apprentice, the chattering powder monkeys of HMS Indomitable, teenage vision Tadzio in Death in Venice, Tytania’s fairies or the watchful Miles and Flora, they are ever-present, but why? There’s something about innocence, certainly, but it’s interesting just how often Britten muddies their symbolism, swapping simple purity for something altogether murkier.

There’s an otherness, an uncanny, ungraspable quality to many of these children. How much do they see, know, understand? And how much do we really know of them? These questions reach their peak in The Turn of the Screw, Britten and Myfanwy Piper’s lean, telling adaptation of Henry James’s ghost story. Louisa Muller’s new production for Garsington treads lightly and discreetly through the tale’s tangle of ambiguities and uncertainties, playing deftly with suggestion, illusion and imagination to create something genuinely chilling — a staging of slow-creep menace and sweaty, night-time terrors.

It’s an elegant trick to pull off in Garsington’s glass-box theatre — sunshine and garden views streaming in on every side, all asserting their comfort, their certainty. But Muller and lighting designer Malcolm Rippeth play the long game, drawing out the opera’s semblance of normality for as long as possible. Is that a figure we see through Christopher Oram’s cloudy conservatory windows, or just a shadow? Even Peter Quint’s first coaxing call is so soft you couldn’t swear you hadn’t imagined it. But as daylight gives way to flickering candles, shadows lengthen along with doubts.

Sophie Bevan’s Governess — pretty, impressionable, dangerously sincere — is the sheltered innocent of this period production. Her eager earnestness feels unworldly when set against the poise and insight of Leo Jemison and Elen Willmer’s children. The maturity of their singing (intonation, expression, phrasing all near faultless) further adds to the sense of miniature adults, whose ‘false little lovely eyes’ conceal far greater understanding than they ought.

Ed Lyon and Katherine Broderick make a sinister pair of corrupters. Lyon in particular, his charismatic, persuasive Quint far more malignant than any grotesque, sings with a disarming freshness, and Muller’s decision to cast him unambiguously as the children’s guardian in the Prologue adds another welcome layer to the work’s ambiguity. Richard Farnes and the reduced forces of Garsington Opera Orchestra arm them with every atmospheric glint and icy gust from Britten’s score, vividly descriptive through every variation, propulsive through every scene.

Can we be certain of the children’s corruption? Thanks to Bevan and Muller’s careful surge of hysteria, of neurotic self-deception, we just can’t. Rarely has the Governess’s closing question — ‘What have we done between us?’ — implicated its audience so directly. The final turn of the screw here, even as we watch Muller’s inspired dramatic coda (no spoilers), is that we ourselves are twisting it, delighting in the chills, willing the story on to its horrible conclusion.

Banishing any lingering horrors, a reminder of Britten’s skill not only in dramatising children but also composing for them, is English National Opera’s latest off-site project. If community opera Noye’s Fludde feels awkwardly crammed and crushed into Stratford’s proscenium-arched Theatre Royal (the work was originally intended for a church), its music finds a comfortable fit in the borough’s young performers. This vibrant retelling of the story of Noah’s Flood brings young instrumentalists, recorder and handbell ensembles, troupes of children, teenagers and music students together with professional singers and actor Suzanne Bertish (God) to create an unclassifiable work that’s part-pageant, part-ritual and entirely charming.

There’s a deliberately rough-hewn, homemade quality to a score whose first raindrops are voiced, in an inspired innovation, by a row of slung mugs, and director Lyndsey Turner and designer Soutra Gilmour wisely preserve this. Illustrations by children’s author Oliver Jeffers provide an artfully naive, two-dimensional arkful of animals (including a tardy tortoise, who arrives late, suitcase in hand), gleefully voiced by local schoolchildren. Their older colleagues are Noah’s sons and their wives — a dysfunctional Biblical family governed by Marcus Farnsworth’s gentle, warmly sung Noah and Louise Callinan as his petulant wife.

Britten doesn’t forget the audience either, who must join in the hymns that punctuate the action. Song sheets in hand, we did — a raggedly enthusiastic, approximate chorus, dredging up distant memories of school assemblies. It wasn’t polished, but it was heartfelt, shared. Apparently the children taking part have been singing their parts in the street, in the playground, at home. Britten knew a thing or two about composing for children. Above all, unlike so many of today’s well-intentioned, apologetic education projects, he knew that, done right, classical music is its own best PR.