This fascinating post by Julie Sedivy in Discover magazine tells us a lot more about Xhosa and its clicks but also makes a number of interesting points about how we perceive sounds. Sedivy points out that to an outsider, the clicks of Xhosa sound "a bit like highly-skilled beatboxing, mixing recognizeable speech with what sounds like the clacking of objects striking each other". Of course, to a Xhosa speaker, the sounds are as familiar as t,s, and d are to English speakers.
In terms of children's acquisition of sounds, the article raises some good points to consider. We often treat the phonological part of a child's language development as mostly to do with the production of sounds and patterns of "errors" in those sounds (deletion, consonant cluster reduction, deletion of unstressed syllables, for example), at least I know I do when I teach it. But what about how we hear sounds as children and how we process those sounds and create a mental representation of what they mean?
Sedivy tells us that "your brain cares as much about how sounds function in a language than about their actual physical properties, so the same acoustic input can be interpreted very differently by the brain depending on its role in a language", so when toddlers are listening to speech all around them, they're not just parroting or imitating it, but making sense of how sounds are used, where they appear and what they might mean. It's as much to do with fathoming out meanings as it is to do with trying to replicate the production of the sound.