Child-Suitable Alternatives To Passwords?
Sorry, but no. You're in the right about wanting to know WHO they're talking to, but not what they're saying. Having been a child fairly recently, I can tell you assuredly that my parents being open about reading, or eavesdropping on, any communication between me and my friends that I didn't explicitly allow them to would have resulted in far worse than automatic resentment. They were open about what they wanted to know: where I was, who I was with, what I was doing, and when I would be home. In return for being reasonable, they got honesty, every time. Yes, even when it meant I got punished. When I was out from under there roof and off to college I was more able to realize just how good I had it. It was the mutual respect between me and my parents that inspired me to get a job during school and stop leeching off of them, even though they gladly would have allowed me to. The kids whose parents' didn't show respect to from an early age are the ones who call up mommy and daddy for every dollar and generally act like the insolent little twits that their parents told them they were. Respect is the key.
That said, the one thing that really bother me about your post is this lingering question about what exactly about an electronic diary means it needs to be monitored (assuming it's not published on the internet). Same goes for e-mails versus other types of communication. What is it that makes invasion of electronic privacy acceptable? Maybe you see a difference between writing with a keyboard and writing with a pen, but I don't, and neither do your kids. Do you listen in on their phone calls, too?