New research says depression is the number-two mental health problem among kids. Here's what to watch for.

According to the World Health Organization:

• Depression is the second most common childhood mental health problem.
• One in 33 kids is depressed.
• The number of kids diagnosed with the disorder could double by the year 2020.

Scary, huh?

Even scarier is that less than a quarter of the 12 million kids in the United States who suffer from mental disorders receive treatment, which makes them more prone to lower grades, drug and alcohol abuse and criminal behavior. Kids with untreated depression are also 12 times more likely to commit suicide.

Many parents don't seek help because they fear their kids will be stamped with the "mentally ill" label, while other parents dismiss their child's behavior as a phase.

How can you tell the difference between a kid who's moody and one with a mental disorder?

Here are three signs your kid is depressed, says child psychologist Fran Walfish:

Change in eating or sleeping patterns: Taking more naps, not sleeping enough, going on eating binges, starvation diets -- all are red flags.

Spending more time alone:  If your talkative tween has turned into a hermit (i.e.: friends stop calling, hanging out in their room too much), it could be a sign they are withdrawing from the world.

Moodiness: Moping around the house, listening to sad music, or saying, "Life sucks" are all warning signs. This may sound like your typical tween, but Walfish urges parents to take any verbal statement seriously, and be on the lookout for signs your kid feels life is hopeless.