Most kids (including my three year old) cannot pronounce certain sound. These might include th or f or v. This got me thinking if English was particularly difficult to learn as a child or if all children in any language struggle with phonetics? Is any one language easier on the mouth and tongue of a child?
The human vocal tract is capable of making over 100 distinguishable sounds (don’t remember the exact number). But no one language uses more then about twenty or thirty.
When you are learning to speak, the muscles in your vocal tract develop according to which sounds you pick up from your environment and strive to imitate. If sounds originating from a particular point of articulation are not included in the languages you are speaking, the muscles that would have been associated with those sounds fail to develop. This is one reason why people who learn a second language as adults often have unshakeable accents.
So, without any actual evidence to back me up, I would venture to guess that childhood speech impediments are pretty much equal-opportunity pests.
Ubykh has 86 phonemes (84 consonants and 2 vowels). !Kung has 147 phonemes.
Rotokas has only eleven phonological segments, and Piraha can be analyzed as having as few as ten. Piraha doesn’t have any liquids, which are typically difficult for children to pronounce.
I stand (well, sit, actually) corrected. Jeepers!
I’ve visited the Netherlands a lot. The residents almost invariably speak three languages. 
However they don’t have the ‘th’ sound in Dutch, so pronouncing ‘the’ or ‘thing’ can be a little tricky.
Here’s an example of Ubykh, if you’re interested. It really doesn’t sound as harsh as it’s reputed to be, though the lack of vowel inventory is really noticeable.
My first impulse on hearing things like this is to ask “Why?”, although, of course, there isn’t a simple “why?” to things like this.
86 consonants and 2 vowels? What are the two vowel sounds?
147 phonemes? Spelling must be… interesting. If I recall !Kung is not traditionally written by its native speakers, but I’m assuming someone has come up with a writing system for it.
It’s theorized that the reason for so many consonants and so few vowels in many Caucasian languages, is that over time various attributes of vowels have migrated to become attributes of consonants instead.
Basically “open vowel” and “closed vowel”, though there’s a whole range of allophones for both of them depending on surrounding consonants. Wikipedia’s page on Ubykh phonology has a pretty useful table.
I’ve only ever seen !Kung written with the IPA.