It’s funny how the beloved movies of one’s less politically correct youth turn out to have a lot more edge to them once you show them to your own kids.

“Back to the Future” has more sexuality than you remember, and little blasts of profanity. “Adventures in Babysitting,” “Bad News Bears” and “Goonies,” even more.

“Fun Size” is in that tradition — at least in terms of the naughty stuff that tweens and teens will snicker over.

Pity it isn’t as much fun as its title implies.

Victoria Justice jumps from Nickelodeon to the big screen with a PG-13 romp that only rarely romps, a movie that surrounds the lovely 19-year-old with funny people and struggles to find them laughs.

Justice (TV’s “Victorious”) plays Wren, a Cleveland high school senior dreaming of the day she can slip off to New York and college, which is where her late father taught her that “you find out who really are.”

First, she’s got to talk Mom (Chelsea Handler, given nothing funny to do) into letting her apply to NYU. Mom’s a bit distracted. Her grieving for her late husband has taken the form of dating/ sleeping with a much younger, goofier, oddly named Keevin (Josh Pence).

And Mom is determined to hang out with Keevin’s loser friends on Halloween, which ruins Wren’s plans to hit the hot high school party that night with her hot-to-trot pal April (Jane Levy, amusingly on the money). Wren has to baby-sit her silent-but-deadly 8-year-old brother, Albert (Jackson Nicoll), whose pranks are epic but who basically stopped talking when their dad died.

The romantic entanglement of the evening is Wren’s desire to hook up with musician Aaron Riley (Thomas McDonell), the “god, stud, legend” who is throwing the party. Meanwhile, her nerdy true-blue pal Roosevelt (Thomas Mann, of the raunchier teen farce “Project X”) has little hope of making time with Wren, dressed as a tarty Dorothy from “The Wizard of Oz,” because he thinks a costume based on biologist E.O. Wilson is a pretty cool idea.

And nobody is making time with anybody thanks to Albert’s getaway. The plump (“fun-size”) kid is dressed as Spider-Man, with one arm just a bloody stump. He escapes his sister’s care and has many adventures involving pranks and assorted run-ins with thugs, girls out clubbing and the like. Many of these misadventures are with Fuzzy (Thomas Middleditch, funny), a mop-topped convenience store clerk who’d be more at home with his best bud Scooby-Doo. Fuzzy explains to one and all that he has an 8-year-old boy in his car.

“Dude, that’s messed up.”

Even though we know where most of this is going, Max Werner’s middling script is sprinkled with surprises — some of them rude, others downright crude. Houses are egged, a Volvo is “violated,” fart jokes abound and Roosevelt’s “moms” (Ana Gasteyer is one) score a couple of big laughs.

But Justice does nothing here that would make her stand out from the current crop of pretty young things trying to jump from TV to the movies.

And TV director Josh Schwartz hasn’t learned the “funny lens” (extreme close-up) or “faster is funnier” rules of big-screen comedy. “Fun Size” waddles along at half-speed, never building momentum. Even the good gags are robbed of their punch by the pedestrian way this thing was shot and cut.

He does better with the sentimental stuff. But the movie’s not titled “Sentimental Size,” is it?