Simmer down, now.

This derivative, dumb and ditzy body switch comedy that ups the “Freaky Friday,” “13 Going on 30” and “18 Again” ante by serving up a whole “Family Switch” is harmless.

No, it’s not the least bit original. It’s cheesy to the point of lactose intolerant, which is another way of saying it’s loaded with fart jokes. And yes, the best laughs reside in the outtakes under the credits. Which include fart jokes.

It’s a holiday comedy directed by the once formidable/still-competent McG (“We Are Marshall,” “Charlie’s Angels”) and could have been scripted by an AI “family holiday comedy” program — but wasn’t (some fraud got credit for writing a children’s book this is based on).

But Jennifer Garner and Ed Helms? They show up and give it their all. And if the situations are laughably unoriginal and the jokes aren’t top drawer, at least they’ve got license to mock that.

“This is a completely unique and original situation, that has LITERALLY never happened before,” our quartet of switched parents and kids (Emma Myers and Brady Noon) kvetch.

“Right? Like no kid has ever woken up ‘Big.'”

“I’m 18 again!”

“I’m thirteen going on thirty!”

The gist? An over-booked, family-ties-slipping LA brood consisting of a “robotic” architect mom, a high school orchestra teacher dad fond of running the string section through “Seven Nation Army,” a soccer prodigy with U.S. Women’s National Team aspirations and a brilliant but bullied son who might skip a year or two of high school and go to Yale get a “one thing you need/fix what is broken” moment from a fortune teller.

Hey, she’s played by Rita Moreno, so who cares if this happens at a Griffith Observatory “planetary allignment” mumbo jumbo accident? You’ve got to go with it. Or your too-young-to-have-seen-“Freaky Friday” kids do.

A couple of days of high school coping, soccer match audition, Yale interview, “big presentation” at work and Dad’s “Dad or Alive” band possibly making it on an “Anybody Left Got Talent?” TV show (Howie Mandell cameos), a Moms wine and dinner part and a high school rave?

No sweat.

Yes, it’s every bit as dumb as every situation there seems. There’s even a baby and pet pug body switch thrown in that’s more gross than funny or fathomable. But Garner, no stranger to the genre (“13 Going on 30” is one of the most adorable body switch pics) and born-to-overperform Helms bring their dancing shoes for a couple of big numbers.

The kids are a tad more charming than annoying. And as I noted at the top, while time may pass oh-so-slowy as this beast grinds through its gears and ruins a ’60 Camaro SS, it’s harmless.

“Not very good,” but “Too cute to hate” will have to do.

Rating: TV-PG

Cast: Jennifer Garner, Ed Helms, Emma Myers, Brady Noon and Rita Moreno

Credits: Directed by McG, scripted by Victoria Strouse and Adam Sztykiel, based on a book by Amy Krouse Rosenthal. A Netflix release.

Running time: 1:46