A movie about grandparents made for grandparents, “Parental Guidance” might as well have been titled “Granddaddy Day Care.” It provides a bland, predictable vehicle for what might have been the inspired pairing of Billy Crystal and Bette Midler—sitcom quality holiday fare that would be totally innocuous were it not for the now-obligatory presence in “family” fare of potty humor, projectile vomiting and comic slapstick (including, of course, the requisite crotch-punch). Very young children, grandpa and grandma may be amused by the combination of mildly naughty comedy and broad sentiment, but everybody else will find it instantly forgettable.

The script is credited to Lisa Addario and Joe Syracuse (known for their screenplay for the drab animated picture “Surf’s Up”), but the impetus came from Crystal, and he clearly provided much of his own characteristically rapid-fire dialogue. The set-up is as simple as any cookie-cutter seventies sitcom. Helicopter parents Alice and Phil Simmons (Marisa Tomei and Tom Everett Scott) want to go off together to a convention where he’s to get an award, but his trusted parents—who usually baby-sit—are unavailable. So despite Alice’s reluctance, they invite “the other grandparents,” who’ve never been close to the kids, to pick up the slack.

The Deckers are a brash pair. Diane (Midler) is, as one would expect from the casting, an outsized personality, but she wants to seize the opportunity to bond with the children. But the real problem is Artie (Crystal), a motor-mouth who’s for years been the announcer for a minor-league baseball team while yearning to make it to the majors. (How the couple affords such a splendid suburban abode on the salary such a job must provide is a mystery.) But he’s just been fired, and doesn’t want Alice to find out. Anyway, he and the kids have never connected. (For one thing, they understandably find his Borscht Belt shtick incomprehensible.) Nonetheless, the Deckers accept the responsibility.

The result is just about what you’d expect. The grandparents clash with Annie—who’s reluctant to leave—on child-rearing tactics. While she pushes daughter Harper (Bailee Madison) to practice her violin incessantly, Diane clashes with the girl’s imperious teacher and urges her granddaughter to have some fun. While his daughter obsesses over her older son Turner (Joshua Rush), who has a stuttering problem, Artie confronts his speech teacher over her methods. And pint-sized entrepreneur Barker (Kyle Harrison Breitkopf) blackmails Artie repeatedly when he finds out about his misdeeds and demands recognition of his invisible pal, a kangaroo named Carl. There are also clashes over diet—the Simmons ban sugary treats while Artie feeds the kids cake that sends them into overdrive, and there’s an especially lame subplot about the family’s favorite pan-Asian restaurant presided over by a stereotype named Chang (Gedde Watanabe, the go-to guy for such embarrassing roles).

Of course the grandparents and kids ultimately find familial affection—though such methods, if you can believe it, as staging a game of kick the can in the backyard and teaching Turner to rattle off an archival recording of a baseball announcer’s famous outburst at the close of a World Series’ game. And Annie learns to respect her parents’ different approach while making up with the father she always felt ignored her. But there are plenty of sidebars in the script along the way. In a gag stolen from Jacques Tati’s “Mon Oncle” but badly flubbed, the Deckers are confounded the Simmons house, which is equipped with all sorts of modern gadgets. Artie takes Barker along on an audition with ESPN where he makes a fool of himself trying to look cool and the kid pees on Tony Hawk’s skateboard ramp and makes him fall down. In another potty-related episode, Artie accompanies Barker to a public restroom and sings to the kid in the stall, to the shock of everyone around. And there’s the tee-ball game where Artie makes a scene, leading the bully who’s been bothering Turner to wallop the old guy in the balls—but Artie gets even, and more, with the kid fast by responding to the assault by throwing up his hotdog all over the tyke’s face.

All of this is feeble stuff, and it’s made even weaker by the lackadaisical direction of Andy Fickman, who at one point simply stops things to allow for a supposedly impromptu duet between Crystal and Midler and draws cable-TV-level turns from the kids and the rest of the supporting cast. The behind-the-camera contributions are strictly mediocre, from Dean Semier’s cinematography and David J. Bomba’s production design to Marc Shaiman’s nondescript score.

There’s a place for movies like this—but it’s really on a family-oriented cable channel rather than in the multiplex.