Ginger Rogers also has great comic instincts, apparent in many other movies and, as stated before, the main problem with her in this role is that she can't- at 31 years old- creditably pass for a twelve year old. But other than that, her acting and timing are great- except for early on when she was required to behave as an even younger child than 12, and that was the fault of the script.
There are also a number of jokes which are legitimately funny, but don't translate too well for anyone not familiar with movies from that time period. Case in point- when Susan is escorted to the Academy's dance by some of the cadets, they warn her that there's a "Veronica Lake epidemic" and they walk in to see all the other young ladies looking like this:
Oh, by the way... there's a well known scene- which is genuinely amusing- in the '80's TV show Cheers, during which Kirstie Alley's character Rebecca who is supposed to be quitting smoking, hides a cigarette in her mouth to avoid getting caught.