Turn it off! Limit it! Do everything you can to keep your children's consumption of television to the bare minimum! We parents hear this message all year long, and as Turnoff Week gets going Monday, the drums will be beating especially loudly.

But what we seldom get - and need - is solid, research-based advice about when to turn ON the TV.

Let's face it. The vast majority of American families use TV and video with their children (the Kaiser Family Foundation says that 53 percent of families with children 6 and under have the set on at least half the time they're home). Yet parents are still mostly in the dark about what our kids - especially the youngest - can actually understand of what they watch. We think that as long as we expose them to "good," educational programming and public television, we're ensuring their positive development. It's not quite so simple.

As a journalist who writes about media and children, I've talked to scores of families about how they use TV with their kids. Parents have told me that they let their 1-year-olds watch Sesame Street and think that Arthur and VeggieTales are good for their 2-year-olds. Many don't let their children watch TV at all, but turn to Disney videos instead, popping in The Lion King for 3- and 4-year-olds. Others say that they simply rely on PBS, regardless of the show and of their child's age.

You probably think that these parents are doing the right thing - avoiding commercial advertising while selecting programs expressly made for children. For years, that's what I thought, too.

But a flurry of new research says we have more to learn. The problem: We're assuming that our children can make sense of what they watch, no matter how old they are. We're forgetting that huge cognitive leaps occur between the ages of 1 and 7.

Researchers, it turns out, doubt that a 1-year-old can even make sense of the sequence of information on the screen, let alone pick up the wholesome messages in Sesame Street.

There's almost no evidence that children under 5 are picking up on the moral lessons in VeggieTales, not to mention the supposedly character-building themes of many Disney movies. And the children's shows on PBS may be more educational, but that doesn't mean that they're always getting through to young children.

"What seems obvious to a grown-up is much more impenetrable to children," says Marie-Louise Mares, an assistant professor of communication at the University of Wisconsin. Mares is the co-author of a recent study, to be published in the journal Media Psychology, of kindergarteners who watched a 10-minute episode of Clifford the Big Red Dog, a popular PBS show, in which Clifford and his friends interact with a three-legged dog. At first the characters fear the dog, worry that they might get sick from being around him and treat him as if he's different. But after the dog tells them that he just wants to be friends, everyone becomes pals.

The story was clearly designed to teach tolerance for those with disabilities, yet most of the children showed no sign of having extracted that lesson from the show. Instead, some got the opposite message.

One child told the researchers that he learned that "you should be careful ... not to get sick, not to get germs." Children seemed to zoom in on the negative parts of the story and forget the positive ending, which lasted no more than a minute.

You may ask, "Where's the real harm if our kids don't 'get' the shows they watch?" If adults are there to provide context, point out new things and shake their hips to the Wiggles, the worries are few. But for most families, TV becomes a babysitter. Would you knowingly hire a babysitter who, no matter how smart, mistakenly leads children astray?

If the intended lesson doesn't always come through in a show such as Clifford, then what about two-hour movies dominated by conflict, or scenes in which children get lost or parents die? Joanne Cantor, author of Mommy, I'm Scared, a book about the effects of fear-inducing media, says that young children aren't likely to get the broader lessons of those movies, nor even the storylines. More likely, she says, they're absorbing the visual effects alone, and some children, particularly the most sensitive, are taking away only the most dramatic or emotionally charged moments. I've heard lots of parental stories about late nights spent comforting children who were frightened by something they saw in a supposedly innocuous children's show.

Experimental studies can be eye-opening. New research from the University of Massachusetts and the University of South Carolina shows that babies younger than 24 months may not be able to distinguish between regular and scrambled versions of Teletubbies episodes. Other studies have shown that around age 2, children can learn words from video if an on-screen character pointedly teaches new vocabulary by, for example, holding up an acorn and saying, "Here's an acorn!" But a lot of TV shows geared to preschoolers aren't designed that way. They feature words and dialogue that have no connection to what's appearing on screen at that moment, so the meaning is probably going over children's heads.

It's the same with Sesame Street. Many parents of infants figure that if the show's good for preschoolers, it's probably good for a 10-month-old, too. But that's not true, says Rosemarie Truglio, Sesame Street vice president for education and research. The show isn't designed to be comprehensible to children younger than 2.