When her parents left her with Grandpa and Grandma for a weekend last October, Morgan Greenfield was an adorable 7-year-old who happily devoured ice cream and pizza.

They came home to an avowed chicken-finger-shunning vegan.

“Morgan just decided, ‘I’m not eating anything that has to do with animals anymore,’ ” says a gobsmacked Felicia of her vigilant second-grader. “It wasn’t gradual. She was, ‘No, I’m done.’”

In 2009, just 1 percent of the US population classified themselves as vegans and vegetarians. Now 5 percent do, and the trend is trickling down to the sandbox set. Long before they hit puberty, precocious local kids are going vegan of their own volition, influenced by social media and a growing awareness of animal cruelty.

“Young people are tuned in to this issue more than ever,” says longtime vegan Gene Baur, co-founder of Farm Sanctuary, a livestock protection organization in upstate New York. “We’re connected to animals and we’re eating these animals.”

To hear Morgan tell it, her come-to-Jesus moment was one of profound clarity, though she can’t recall if there was something — a TV show, a book or a Facebook post — that inspired it.

“Something popped up in my mind that said, ‘Don’t hurt the animals, no dairy or eggs,’ ” says the youngster, who admits she misses M&Ms.

Morgan’s parents are struggling to keep up with her new diet, and that of her sister, Danielle, 10, who went vegan a few months after Morgan.

Their mother, Felicia, worked with a vegan coach for six weeks to learn to cook plant-based meals.

“I thought, ‘Oh boy, I have to teach her to eat her veggies, and I don’t even know that much about it,’” says the Upper East Side mom, who wasn’t much of a cook and had previously served her kids lunch meats and hot dogs.

‘Something popped up in my mind that said, “Don’t hurt the animals, no dairy or eggs.”‘ - 7-year-old Morgan Greenfield on her sudden conversion to veganism

Still, there’s the occasional hiccup, like when Felicia gave Morgan a snack with red food coloring, which isn’t vegan because it’s derived from tiny parasitic insects.

“She was so angry. She didn’t talk to me for the rest of the day,” Felicia says.

Morgan is also pushing her friends and family to change their diets. She loves hosting vegan dinner parties, where she attempts to convert meat-lovers with veggie tacos and kale salads.

Her dad, Nick, a former sushi addict and steak connoisseur who works in finance, has been vegan since January. But Felicia, who’s long been a vegetarian, hasn’t joined her kiddies’ crusade.

“I couldn’t give up cheese,” she says.

Not that Morgan isn’t still trying to convert her.

“The other day I was starving and bought a candy bar, but she busted me,” says Felicia. “She said, ‘Mom, that’s milk chocolate!’”

Other kids are turning to the restrictive diet for health reasons.

Samantha Arnel, a seventh-grader from Edison, NJ, took it up last fall to help with a kidney disorder she suffers from called Bartter syndrome. She didn’t like the way she felt after eating hamburgers and milkshakes and was inspired by her older sister, Michelle, 14, who is also vegan.

“She did it more for the animals,” she says. “I did it more for health reasons.”

The diet is working. She’s lost 10 pounds and says she feels better. At her bat mitzvah last month, she served four plant-based entrees as well as a chicken option for the uninitiated.

Her parents were OK with the menu, though her dad, Ross, was wistful that he couldn’t celebrate at the country club with meat.

“They have great steak there,” says the father, who works in insurance. He also notes that finding places where the whole family can eat has proven challenging.

“It’s not easy,” says Ross, who has zigzagged across New Jersey to find eateries where the girls can nosh with abandon. “We have to go to a Japanese restaurant and find options without eggs.”

He now mostly eschews eating animal products himself. “If I’m eating at home with them or at a vegan restaurant, I’ll go vegan,” he says. “They’re influencing me.”

But while kiddie vegans might be convincing their parents, health experts aren’t so sure of the restrictive diet for the preteen set.

“I wouldn’t recommend it,” says Danielle Pashko, a nutritionist for Park East Day School on the UES. “Adults eat tofu, tempeh, veggies — they know how to get their B vitamins and amino acids, protein, calcium, vitamin D … but kids aren’t naturally inclined to eat vegetables. Most of the time, a vegan child who’s left to their own devices will have nutrient-void foods — a bowl of Cheerios, pasta or potatoes.”

But resistance can be futile. “They see it as a life commitment — they are hard-core,” says Maggie Siena, a Brooklyn mother whose kids Jacob, 12, and Julia Schles, 15, made giving up eggs, meat and dairy their 2016 New Year’s resolution.

Jacob is especially evangelical about his plant-based lifestyle, even though he’s had to go hungry at friends’ pizza parties.

“I can get a little angry about it, which I need to work on,” says Jacob. “[Classmates] just don’t agree with my perspective on veganism … I’m trying to show them what’s best for the world.”

He hasn’t fully convinced his mom, who’s given up meat and fish but not eggs.

“His birthday wish was, ‘I want my parents to be vegan,’ ” Maggie says with exasperation. “I tell him, ‘Lighten up, Jacob!’”