“It’s not a toomah! “With those words Arnold Schwarzenegger wormed his way into the hearts of millions of teens and tweens (some of them future California voters) in 1990’s “Kindergarten Cop.” The steroidal star of body count films was hardly an obvious choice for the role (it was first offered to Bill Murray), but he made it a cross-marketing smash, raking in close to $100 million and introducing a whole new generation of moviegoers (and their parents) to his Teutonic allure.

Fifteen years later, Vin Diesel is trying the same stunt with “The Pacifier,” a derivative wisp of a movie in which he plays Shane Wolfe, a Navy SEAL who unexpectedly finds himself babysitting a household full of children. This kiddie-action subgenre is a dispiriting one, clearly intended to hook younger viewers on stars who will shortly be returning to their day jobs massacring battalions of bad guys. Soon, no doubt, the Rock will ante up with “The Scorpion Nanny,” before Keanu takes the jackpot with “The Matrix: Reflux.”

As gateway drugs go, “The Pacifier” is tamer than “Kindergarten Cop,” with less strong language, fewer adult themes, and very little bloodshed. Depending on your point of view, this could make the film more innocent or simply more insidious. Either way, this is about as close as you can get to a family comedy when the surrogate head of the family is a trained killer.

The movie begins with an action prologue in which Shane rescues a top U.S. scientist, only to have him be killed (offscreen) as they make their escape. Complicating matters, the scientist’s latest military invention has gone missing. While his widow (Faith Ford) flies to Switzerland to check his safe-deposit box, Shane is assigned to guard her five children, who range from infancy to high school, against any foreign spies who might come to the house looking for the device.

His job description expands somewhat when the children’s nanny (played by Carol Kane as a kind of Transylvanian bag lady) quits, having been vomited on and tripped down the stairs one time too many. It is a tough transition at first for Shane, who undergoes such de rigueur humiliations as changing his first poopy diaper and driving the family minivan. But following a Ninja attack on the household – repelled with talcum powder, broomsticks, tennis racquets, and a plastic play tunnel – Shane begins to earn the children’s trust. In no time, he’s giving driving lessons, performing the Panda Dance, acting as den mother to a troop of Fireflies, and directing a bit of community theater. The children even learn to put up with his insistent use of military lingo.

There are a few humorous twists along the way – one concerning a misinterpreted DVD and another involving “The Sound of Music” – but for the most part “The Pacifier” is dimly conceived and ploddingly executed. There are half-hearted subplots involving the fetching school principal (Lauren Graham, looking as though she hasn’t quite recovered from “Bad Santa”), her oafish, bullying vice principal (the hulking, hirsute Brad Garrett), a squad of miscreant preteen Cub Scouts, and the unfriendly Asian neighbors next door. Apart from such trimmings, the story hews closely to its “Kindergarten Cop” model, with Shane teaching the kids about discipline and self-esteem and them teaching him about the joys of domesticity. Whereas the Schwarzenegger vehicle had a pet ferret destined to bite the baddie at the climactic moment, “The Pacifier” assigns this mission to the family duck.

The biggest difference between the two films is their respective stars. For all the well-deserved knocks on his acting style, Mr. Schwarzenegger had a surprising knack for comedy. With his mobile face and keen willingness to look ridiculous, he was able to draw a distinction between his already cartoonish screen persona and a parody of that persona. (Indeed, two of his first three box-office breakthroughs – “Kindergarten Cop” and the earlier “Twins”- were Ivan Reitman-directed comedies.)

Vin Diesel, by contrast, is a relatively inert straight man. He lacks that Schwarzenegger “wink,” the ability to simultaneously be the joke and be in on the joke. His bewilderment at finding himself in a universe of diapers and cookie sales and trips to the principal’s office seems less comic than actual, as if he’s really unsure what it is he’s doing in this movie. It doesn’t help that with his bald pate and over enunciated speaking style, he almost seems like a giant child himself.

In the end, “The Pacifier” is just what it looks like: an uninspired, formulaic comedy trying to bridge the teen and preteen years. A 12-year-old may find it uproarious. But anyone much older is likely to feel that it fulfills the promise of its title, and sucks.