For the past seven years, the “Diary of a Wimpy Kid” movies have been the little franchise that could — but never did. Why is that? The books, by Jeff Kinney, are major sellers among the elementary-school crowd thanks to their smart-mouth attitude and why-me despair. The movies captured that vibe nicely with a charming cast led by Zachary Gordon as our deeply flawed hero, Greg Heffley. Best of all, the “Wimpy” movies managed to deliver worthwhile lessons in a saccharine-free way.

It’s too bad these movies, though profitable, never caught on in a major way, because now Gordon is too old to play young Greg (the actor recently entered college). He’s been replaced, along with the entire cast, in the fourth film, “Diary of a Wimpy Kid: The Long Haul.” The new actors are passable replacements; if you squinted, you might not notice the difference. The real problem with this film, though, is that it feels like so many other kids’ movies, a cacophony of cartoonish misadventures and bathroom humor.

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Our new Greg is played by Jason Drucker, an amiable enough presence even if his character is more grating than usual. “The Long Haul” begins with a family in crisis: The glow of the smartphone has become more appealing than human interaction. To correct this, Greg’s mom (Alicia Silverstone) corrals everyone for a car trip to visit her 90-year-old mother. Greg and his teenage brother, Rodrick (Charlie Wright, of Netflix’s “Stranger Things”), have other plans: to sneak away and attend a nearby video game convention.

Road-trip movies get by on their gags, not their stories, but “The Long Haul” is short on both. There’s a seagull-poop fiasco, a flatulent piglet, an overweight guy on a toilet, and so on. Meanwhile, the low humor is mirrored by the characters’ low behavior: Greg’s dad (Tom Everett Scott) is lying to his boss about his whereabouts, just as Greg and Rodrick are lying about nearly everything else. The script (co-written by Kinney) seems slightly unclear about what counts as a venial sin.

Directed by David Bowers with minimum flair and maximum chaos, “The Long Haul” seems intended to finally connect with its young target audience. In the process, though, this series has lost much of its charm, and possibly a little bit of its soul.