i think having this double standards regarding the playing of mozart by children and by professionals is true and good to have.
------------------
you see, the innocence of mozart's music is difficult for the adult pianists because we try to make it come alive (with all the dynimics and phrasing, etc.) but we are always careful to keep it at a certain level of simplicity. to keep the simplicity and yet, come out in full colors and imaginations is really hard to combine. that's why we need it to be sparkling in every note (each has to mean a thing) or else all we play are just the pure notes. it is as delicate as chopin's music IMO. and both require a certain personality to be able to pull off well. one has to be charmingly playful but also elegant, well-mannered but also without pretense.
the more simple a thing is, the more difficult it is to do. besides having to achieve technical perfection with a transparent kind of piano writing, what else could you do with just a few notes in order to make it sound interesting?
a rachmaninoff prelude is surely easier than a movement of mozart's sonata. the former requires you most importantly your rhythmic precision and notes, dynamics, etc... if one has the patience and the right tools or technique, then all you would ever need is feeling and fire, backed up by some intellectual level. this you can achieve in time. you can even plan out a time table for it, but surely, you will achieve it. on the other hand, with mozart, a certain "atmosphere" (i'm at a loss for the better word) in one's playing is required of the pianist. without this, the playing will be just purely notes. the most important thing to consider is that we can't really say when or how we can achieve that "atmosphere" coz we don't know how to achieve such.

some pianists just happen to have it.

i'm almost close to saying that playing mozart (and chopin, IMO) requires only some kinds of personalities of pianists. actually, this i truly believe in. i don't say that other composers are "easier" (whatever that means), it's just that there are many ways of playing them and still they will sound great.
i actually have double standards when listening to recordings of mozart (especially chopin): 1) those which are colorful and interesting, 2) those which i simply feel that the "have it".
clara haskil and alicia delarrocha i admire most for their mozart. alfred brendel and murray perahia come second. with chopin, i'm 'crazy for ivan moravec', hehe, and also zimerman, of course. but the former is a more personal touch IMO.
i consider myself a bad player of chopin and mozart.

however, that doesn't stop me from playing them coz i love their works.
