Phonological development of children's pronunciation

Trends in phonological development 

It is very hard to be precise about phonological development and the way in which vowels and consonants are acquired as it can differ massively from child to child. Here are a few of the trends researchers have identified:

• Command of all the vowels is achieved before all of the consonants 
• At four the child is likely to be having difficulty with only a few consonants 
• The child may be six or seven before confidence in using all vowels and consonants has been acquired
• The general, sounds that occur frequently in a large number of words will be acquired before sounds that occur less frequently
• To make words easier to say children will simplify their pronunciation is certain ways like deletion, e.g. final consonants might be dropped like the 't' in 'hat' and 'cat'

Another form of simplification is substituting harder sounds with easier ones:

• R becomes w
• Th becomes d, n or f 
• T becomes d
• P becomes b

Patterns of sound changes
As well as having difficulties with individual sounds, children often use patterns of 'shortcuts' to make words easier for them to say. There are many different types of these patterns - you may have heard the following in a young child's speech:

•  leaving out the final consonant in a word; for example, 'ca' for 'cat'
• dropping unstressed syllables; for example, 'nana' for 'banana'; 'sketti' for 'spaghetti'
• repeating the first syllable of a word; for example, 'bobo' for 'bottle'

These three patterns usually stop by the time a child is three and a half years of age. Some patterns may last for longer, such as reducing sound blends to one sound; for example, 'pug' for 'plug'.

Bibliography:
http://revisionworld.com/a2-level-level-revision/english-language/child-language-acquisition/phonological-development
http://www.kidshealth.org.nz/speech-sound-development