The most effective—but unpopular—way to raise happy kids, says Ivy League child psychologist

The key to raising a happy child is to allow them to be unhappy.

It might sound counter-intuitive, but it's highly effective, says Tovah Klein, a child psychologist and author of the book "How Toddlers Thrive."

"We all think the way to raise our children to be happy is to make them happy. But in truth, children know how to be happy, to find joy. It is not an all-the-time feeling," Klein, director of the Barnard College Center for Toddler Development, tells CNBC Make It. Barnard is an undergraduate women's college of Columbia University.

Parents often have trouble accepting that "children are allowed to be angry, sad, unhappy in some way," she adds.

When a child is upset, parents often instinctively look to cheer them up or distract them. They might offer up chocolate cake, or go to the park to run around. There's nothing wrong with doing nice things for your kids when they're unhappy, but it doesn't always address the source of what's upsetting the child, Klein says.

That's especially true when the source seems trivial — like not being allowed to watch another episode of television or seeing the "wrong" brand of cookies on their dessert plate.

Even in those cases, the child believes their negative reaction is appropriate, and they need to learn to address and manage those feelings — which they don't intuitively know how to do, Klein says.

You could ask an upset child to take a deep breath and attempt to put their feelings into words, experts say. Or, acknowledge the child's feelings, even if you have to stand firm, with phrases like "I wish we could do that. Unfortunately, we can't right now," Klein advises.

"Strength comes from being able to have these pretty intense emotions, like anger, [then] handling it and knowing that 'Mommy or Daddy is still there for me, they're not upset with me, they're not going to cast me aside," she says.

Most importantly, understand that the child's negative emotions will always pass, barring the presence of any major traumatic events. "Assuming that their life is OK, they're going to be happy," says Klein.

Children who learn to manage their negative emotions effectively are more likely to develop the resilience they'll need as a successful adult, research shows.

A fear of those negative feelings can lead to long-term behavioral problems, Klein says. Kids can become ashamed of those feelings, and have thoughts of self-doubt like, "I'm angry, I must be bad. There must be something wrong with me."

That's exactly why parents need to accept that their children can't be happy all of the time, she adds: "That's the hard part for us as parents: We're happiest when our children are happy. Who wouldn't be?"

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