These cases may be rare, but it often takes only one troubled child to start a dangerous trend among peers, said Dr. Susan Lipkins, a psychologist who specializes in school conflict.

“They’re acting out, needing attention, and there may be some reason,” Lipkins said. “There may be something in the child’s life where they have already been exposed to sex, either by seeing things or hearing things they shouldn’t, or perhaps being abused.

“One child who acts out will be the one with the problem, and the others who are watching may follow.”

Dr. Rebecca Bailey, a family psychologist, agrees.

“There’s a lot of double-daring going on,” said Bailey, author of “Safe Kids, Smart Parents: What Parents Need to Know to Keep Their Children Safe.” “I knew a boy [around 11 years old] who did it to make the other kids stop thinking he was nerdy – that’s a fit-in thing.”

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That kids not yet in their teens would send sexually inappropriate text messages is no surprise, she added.

“With kids getting periods as young as 9 now, there may be some that are a little more aware of sexuality,” she says. “They’re also practicing intimacy on some level, too. Some of these kids are ‘going out’ with each other at 11, whatever that may mean. What we’re talking about is wanting to fit in, maybe being somewhat impulsive, and figuring out their identity.”

But even younger children who aren’t thinking about sex are aware what they’re doing is inappropriate, Lipkins said.

“I think people are apt to think ‘oh they didn’t know what they were doing is wrong,'” she said. “They knew; they just couldn’t predict the consequences.”

Experts agreed parental talks about sexting should start young, even before middle school.

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Exactly what you say to your children depends on their age – and not necessarily the one that’s displayed on their birthday cake.

Parents need to “understand developmentally what age your child is at,” Bailey said. “Some 11-year-olds are still 9-year-olds. That can vary in a house. You can have twins with different developmental ages.”

Here’s how to start the conversation with your younger child.

Start with the basics. Elementary schoolers can be taught phone safety rules that will help prepare them for peer pressure later on. “Even a younger child can understand, don’t take pictures of anything that’s private, don’t expose your last name or any identifying info,” Lipkins said.

Give them a “dumb” phone, not a smartphone. “The phone you give your child should only have features that child is ready to handle,” Lipkins said. If a child can’t exercise good judgment with photos or texting, parents can and should disable those functions. Most elementary-aged kids only need a basic phone with mom and dad’s number programmed on it, Bailey said.

Have access to the phone – and use it. “Parents should be able to see texts and photos, and have the passwords” to any apps, Lipkins said. Privacy “can happen later during high school,” she added. “In middle school, kids still need supervision.”

Ask questions instead of lecturing. “Be a bit of a detective” to find out what’s really going on in your child’s life, Bailey said. “Allow them to teach you,” she said. “Use examples about other kids that you know. Often that will give you more information than hitting them head on.”

Don’t stop at one conversation. “It’s never a one-stop shop” for this kind of parental guidance, Lipkins said. Set down ground rules before you give them the phone, then check in periodically.