What may be special about kids learning a language (or anything, really), is that they have no preconception of a 'correct' way to do things, including how to move their tongues.
That being said, I can't whistle to this day.
When I was younger I knew a girl whose voice I did not understand when I first met her. It took me some time before I was able to understand what she was saying, because I hadn't heard a voice like that before. Based on that event, I think that the reason people can't hear the specific details of pronunciation is because they're not used to it, but they can learn with more experience.
'Course it would then take further training to be actually able to speak them.
Then they got vowel exactly correct by mimicking in English one of the English language accents!
The various different English accents cover a huge variety of vowel sounds, and quite a lot of consonant sounds.