Talk about reporting the bleedin' obvious.
Everyone knows that kids don't talk about "what we did in school today" - at least, when they get past the grand old age of 6. How do we know this? Because we did it ourselves, plus our parents did the same and no doubt their parents before them.
Part of the reticence is about power: there are few areas where kids have any control, and having your parents ask you for information is obviously one of them. However, part of it is also about intrusion. A school is a place for the kids (despite what the teachers might think). If their parents start intruding into that place - either by turning up there and seeing for themsleves, or by giving their kids the third-degree, there's bound to be resentment and resistance - afterall, the kids don't ask their parents "what did you do at work all day" (or, worse: "what did you do sitting around at home all day"?)
However, if they really want to know, and I suspect that if they were ever told the unexpurgated truth, they'd quickly learn never to ask again, there should be a quid-pro-quo. After all the kids should be able to get something for this seemingly valuable information. If parents are so anxious to find out what goes on, their kids could well ask "What's In It For Me?" Let's see how much they're prepared to pay for it.