When Bronx mother of two Sirin Samman walked into her daughters’ room recently during a “time-out,” she was shocked by a “to-do” list her 8-year-old had written. Her older daughter, Maia, was so upset about being locked in with her 6-year-old sister, Nayla, that she wrote the following: 1. Clean room (my instructions), 2. Make Nayla miserable, 3. Be mean to Nayla, 4. Pick on Nayla, 5. Terrorize Nayla a bit.

Samman was amazed that her “incredibly sensitive and compassionate kid” could be so nasty. It’s a feeling many parents have experienced lately as the culture of “mean girls” becomes an increasing nationwide problem.

“Tiny Monsters,” a new animated series now in development for Fox, aims to reflect this growing trend in schoolyard culture. “At the age of 12, [girls reach] the apex of their power,” says the show’s executive producer Ali Rushfield who, along with high school pal Jack Black, sold the show as a “ ‘South Park’ for 12-year-old girls.” Says Rushfield: “That’s as powerful as you get until you potentially become like Hillary Clinton.”

With the government holding its first ever bullying summit this past August in Washington, DC, and the American Academy of Pediatrics Council revealing that kid cruelty online is “happening more than people want to admit,” the disturbing statistics are also on the rise. In 2007, 32 percent of students between 12 and 18 said they were bullied — up from 28 percent in 2005, according to the Department of Education.

And girls are particularly bad, say experts. Theories abound as to what’s causing the cattiness, from early onset puberty to anonymous Internet cruelty to societal narcissism to the witty “mean” banter on Disney shows (note the constant refrain of “Whatever” on “Hannah Montana” or “Shut it!” on “Sonny with a Chance”). But one thing is for sure: It’s happening younger and more viciously — and many parents are left shellshocked by the schoolyard tales.

“When I gave birth to a little girl, I knew mean girls would eventually be our cross to bear,” says Jeanne Sager, who lives in the Catskills and is a writer for The Stir, a blog on the CafeMom site. “Little did I know it would start in preschool. My daughter and her friend would both come home relating that there was a group of girls who would not allow the two of them to play with them. That group of girls referred to themselves as ‘The A Girls.’ At 4!”

Do you think today’s kids are getting meaner?online survey

Fellow mom and writer for “The Stir” Sasha Brown-Worsham adds, “My 3-year-old told me that a boy in her class said she is not as beautiful as another little girl. When I asked her how that made her feel she told me that he was a ‘stinky boy’ and that his mother was stink, too. I swear to God, I have no idea where my child picked this up. It was like she invented ‘yo mamma so fat’ jokes. I have never said anything like that in front of her.”

Ann Lane, a nonprofit administrator in The Bronx and mother of 8- and 5-year-old daughters, frets that “shows like ‘Rugrats’ from long ago created a prototypic social cunning in young girls that we see today in ‘Gossip Girl.’ ” Says 31-year-old mom Becky DiStefano, “I’m officially scared for my kids.”

“Our middle schools are now filled with miniature Medeas,” observes feminist theory professor Gina Barreca, who teaches at the University of Connecticut. “Your average little girl makes Vlad the Impaler seem as empathetic as Oprah.”

But mom Stephanie Klein thinks that we’re missing the boat in blaming society or pop culture. “If kids are getting meaner younger, it’s because they’re getting everything younger,” observes the mother of almost 4-year-old twins and the author of an upcoming memoir about how the Girl Scouts saved her life. “My 8-year-old niece has boobs. If I had to start wearing a bra before I could see ‘Harry Potter’ without parental guidance, I’d be a bitch, too.”

Lifestyle coach David Wygant highlights how reality shows are encouraging kids to verbally outsmart each other while at the same time we’re teaching kids to play mind games at younger ages. “Psychological [attacks are] easier to disguise and more socially acceptable,” Klein says. “You’re white trash if you go for her ponytail.”

Comedian Elon Gold actually blames the Internet, plain and simple. “Now with the press of a button, kids have friend-nixing power. To accept or ignore, that is the question of the day. Rejecting is mean. Ignoring, that’s just soul crushing!”

Even celebrity dads are horrified by what they’re witnessing. “Sex and the City” and “Californication” star Evan Handler witnessed his 4-year-old girl being ignored and taunted by another toddler. “I had to explain to her that sometimes kids are mean,” Handler said, at a recent Cinema Society party. “Eventually I told the little girl to f – – – off. In my own gentle way.”

Overall, though, parents agree they have to work harder to be nicer in order to combat the mean girl culture. “Mean girls beget mean girls,” says author Rachel Maude, 31, of the “Poseur” young adult series. “I once witnessed a mother in a Manhattan shoe store push her toddler up to a mirror, and sneer, ‘Look how ugly you are when you cry.’ If we want nicer children, we better start behaving ourselves.”

mstadtmiller@nypost.com