Guys, don't get me wrong here. I'm not talking about idolizing Diana but if children wanted to know who she was, then I think it's okay to tell them about her and the charities she helped support. That she wasn't a saint or anything but she was a compassionate person who used her status as a senior royal to do good towards others. That could be one way to keep her memory alive.
I agree with your point here.
I disagree with the general idea that Diana's legacy is fading because her memory is fading. Look at the continued attacks towards Camilla on the DM - that's part of Diana's legacy. Look at how people jump every time Catherine is doing something like Diana - also part of Diana's legacy.
The problem with Diana's legacy, as I see it, is that when she's pulled out in the media it's never as Diana the Person, but rather as Diana the Saint.
We often see William, Harry, and Catherine being compared to Diana and while that's good in theory, in practice it's flawed. When the boys get compared to Charles or other family members it's more of a "William is like Charles in this, and that's good/bad," "Harry does this differently from Charles and that's good/bad," or "the boys are like this, which is different from how their dad does it, but both ways work." The comparisons are even handed and aren't made necessarily to tear down anyone.
When comparisons are made to Diana, however, it's more often a comparison to the mythical Saint Diana and not the real Diana. As a result, either the boys are built up because they're like their mother, or torn down because they aren't like her. Similarly, Catherine's built up and torn down depending on how much like Diana she is, but she's also built up and torn down based on how much the media thinks she's imitating Diana and whether or not they think that's a good thing at any given time. As we see William, Harry, and Catherine as more rounded individuals, we're placing them in what is essentially a game of roulette that they're destined to lose.