Dressed in Ku Klux Klan pointy-hatted smocks, rainbow-striped socks and hippy sandals, an audience of mannequins stalk Jake and Dinos Chapman's Come and See at the Serpentine Sackler Gallery. The gallery is filled with familiar horrors: giant vitrines heaving with piles of corpses and a holocaust of tiny human victims. There are zombies, Nazis, Ronald McDonald crucified dozens of times, rampaging dinosaurs and a pair of unpleasantly hairy human legs, reproduced at a god-like scale. We might have seen all this before, but the excess and labour and attention to detail is still a wonder.  Then there are the tabletop brain experiments, jury-rigged with hammers, glistening cerebellums, bottles of goo, power tools and tubes. These mad torture-decks, sticky with unnameable juices and given a liberal sprinkling of maggots and mealworms, are ossified in distressed, patinated bronze. Somewhere among them, the heads of Jake and Dinos suck at the breast.  As well as a flock of stuffed crows, old sculptures and new, plus delicate recent drawings filled with spidery whorls that look old – as if scavenged from Hans Bellmer's dustbin – the walls are covered with etchings, paintings, art-school life drawings by Jake (Dinos burned his) and lines of vinyl text, like a hyperbolic cosmic gush written by HP Lovecraft or William Burroughs in sci-fi mode. The text is portentous drivel. How many cosmic hurricanes spraying out into the void can one take? This sort of thing rattles some of the Chapmans' commentators. Serpentine directors Julia Peyton Jones and Hans Ulrich Obrist say in a statement that the Chapmans "compel us to confront the nagging fears that lie at the dark heart of the western psyche". That's one way of looking at it.  Entering middle age, the Chapmans are no longer the enfants terribles terrible they perhaps once were, however infantile the humour might appear. Yesterday, I asked Dinos when they were going to grow up. "Never!" he replied. Horror-movie schlock is but one of the artists' modes. The Chapmans' art is enjoyable rather than shocking. In the end, you focus on their demonstrable craft, drawing, production values and other misdirected skills.  The Chapmans are very good at what they do, even when they do things badly-on-purpose. They always go too far: but too far is not always quite far enough. How far can anyone go in the name of art? The most upsetting is often the quietest and least dramatic. The rest is a circus and a charade. Real shock lies elsewhere, outside the gallery. The Chapmans know this.  More and more, I feel that their best things are the smallest. One of the cave-like gunpowder rooms in the centre of the gallery is filled with table-top arrangements of cardboard dinosaurs and models of earlier works and exhibitions, complete with little painted card spectators. One paper cut-out woman has fainted. These small, collaged paper-sculptures have a great feel, sense of touch and play. Everything they do comes over as a silent snigger.  The other powder room has been decked out as a cinema, where a hilarious film plays, cobbled together from an early video made in the studio, and the 15 minutes or so of the 2010 film The Organ Grinder's Monkey, all that the Chapmans managed to complete of their commercial debut before time and money ran out. It's a spoof life-of-the-artist movie, starring Rhys Ifans and David Thewlis (with Daniel Craig dressed in a gorilla suit). There is masturbation, cockroaches, there is filth, abuse and dank humour. We even revisit the Chapmans' old art school, where a life-drawing tutor, played by Thewlis, remembers the pair as diligent, affable, "passable" students. In another vignette, the brothers themselves appear in cameo, emerging from a giant vagina, given birth by Samantha Morton. It is all very gooey and despicable, and very funny.  Will children be horrified, corrupted, or given nightmares if they see this show? Adults might have a bit of explaining to do, but there's nothing so nasty here as the eyeball-slicing scene in the 1929 surrealist masterpiece Un Chien Andalou, let alone on the TV news. Kids are not so innocent, and know the difference between life and art. They might just want to go home and make a Chapman for themselves.
These are just some of the many despicable, gooey and very funny things on display at the latest show by art's enfants terribles as they step into middle age, writes Adrian Searle