Time was, kids would come home to show off their latest test, adorned with a star from the teacher. At the dinner table, they talked about what happened at school -- or, in response to questions, sullenly said, "Nothing," leaving their parents to worry about how badly things had gone. Now they just give you the password to the browser.

Who needs maternal instinct? Today, the school's online data systems tell me everything I need to know about my children's classroom performance. From my desk at home, or work or via Wi-Fi, I can find out whether they turned in their homework, whether they cut class, what grades they got on the tests they said they didn't need to study for -- and, in a new twist, how many cookies they had for lunch.

I discovered my new spying ability the other day while depositing money through the Internet in my teenage son's school lunch account, something you do instead of handing out cash or slapping together peanut butter sandwiches. Conveniently linked to PayPal so I can use my most familiar password and rack up more JetBlue miles while feeding my adolescent offspring, the Online LunchBox acts as a cafeteria debit account that my son can access by punching in his PIN at the cashier.

Nice. But this time, I noticed more. The LunchBox allows me to set limits on how much money Sam can spend each day, and even some restrictions on what items he can buy. And in case I harbor suspicions about how he might be spending the money intended for his nutrition, it also allows me to examine everything he's been ordering, day by day.

This is almost as intrusive as the tracking system at my local supermarket. Thanks to Aeries, I already know what Sam's Spanish teacher thinks of his class participation. Isn't that enough? He's 18, heading to college, where he'll do completely embarrassing things that I thankfully won't see. He's received only one speeding ticket so far. At this point I'm going to check his purchases of popcorn chicken?

Yes, I am. And among the daily orders of lunch items and a drink, I find some interesting food choices and budgeting decisions. Like the day Sam bypasses lunch but instead grabs three giant cookies. There also are repeated references to a $3.75 item marked in ominous, all-capital letters, NAKED. There are days in which NAKED constitutes an entire lunch. Or NAKED and chips. This cannot be good.

What do I do now? Use my newfound online power to limit the cookie buys and leave him to wonder when the cashier shuts him down? Admit to him that Big Mother is watching his snacking habits? Demand to know the identity of NAKED?

Now that technology -- global positioning, at-home drug tests and online grading systems and cafeteria records -- can tell parents so much about their children, the question necessarily arises: How much is it good for us to know? These information systems are useful, but we kid ourselves if we don't acknowledge each one comes with the price of eroding, if only a little, children's rich private lives.

Twenty years ago, a big topic of parental discussion was the loss of neighborhood playtime. Most children no longer could yell at the front door, "I'm going out to play!" and disappear for a few hours to whatever secret enjoyments they preferred -- sometimes doing nothing but simply relishing moments out from under adult scrutiny. These moments were replaced with parentally scheduled and supervised play dates, or wholesome, structured activities -- music lessons, league sports. Ever since, the parental noose has been tightening on children's privacy.

If I can see the grade on every homework assignment, I can warn my children that a missed paper earns a zero, and even when averaged against a 100%, that's a flunking grade. But maybe it's not so terrible if they live a few weeks with that sudden discovery, made on their own, along with the lurking fear of what will happen when my husband and I are finally clued in by the report card.

As a child, I filched my mother's empty jars to raised a horde of caterpillars that turned into moths so terrifyingly hideous that I rolled their jars down the street to break into dangerous shards and ran away with the secret of my crime, punished only by weeks of nightmares. A friend lived for months with the worry that she would be hauled off to jail for her overdue library books, which she kept hidden lest her parents discover them. Those fears and secrets are childish, of course, which is their very point. They provide children with an existence of their own, beyond adult realities and lessons.

Like any parent, I desperately want to save Sam from -- or threaten him about -- any awful, life-changing mistake. Yet I want him to gain the strength and maturity that come from making those mistakes, and I have to remember they don't have much to do with whether he buys chips for lunch.

In the end, what's most important is for my children to understand my fundamental mantra of life: At age 21, you are responsible for your own health insurance.

Sam and I had a brief chat about the lunches. NAKED turns out to be, of course, designer fruit juice. We agreed that, because the food budget is my bailiwick, I would limit the amount the account would pay each day for lunch. Within that, he gets to exercise his nutritional options, or add to the daily amount with his own cash. He'll make some bad decisions. I hope they'll be good for him.

Besides, his younger sister is still in middle school, the stage at which they don't yet consider a lunch from home a social humiliation. So her lunch drink is water; the bread, whole wheat; the side dish, carrot sticks. As I count out the number of Pringles allowed each day (five) in a reasonably sinful lunch, total control is once again mine.

Karin Klein is a Los Angeles Times editorial writer.

karin.klein@latimes.com