That's their individual prerogative. But they don't have ownership of the symbol (and, yes, I have several Vietnam vets in my family and yes, I have young nieces who wear what they consider "cool" retro clothing with tie dye and the peace symbol and no, none of the Vets in my family see anything offensive about it)
Parents have the responsibility to teach their kids what the symbols mean. I believe that the 0bamas have taught Malia what that symbol means and the fact that she is wearing it on the same trip her father discussed reducing nuclear weapons with Russia cannot be taken a a coincidence unless you think the 0bamas are completely stupid about symbolism which, we know well, is the opposite of their demonstrated interests.
Just 'cause young, dumb kids wear something they think is "cool" is no reason to ignore the meaning of those symbols and just allow them to wear them in public, especially on official govt trips.
Ignorance is NOT an excuse. What if it were a swastika? Would your "cute, innocent" tolerance work then? After all, she didn't know about that old symbol, unless her parents told her about it...