Here's some info from a useful publication ( you can download this on the web) called "Much more than words" that the Ministry of Education/ Special education division put out a little way back. It's a user friendly look at key ideas of communication development, when there is a problem, and ideas to promote skills. If you want a copy of this you can download it or ask the team to email you a copy. We've had some families talk to us about speech sounds and the content of their child's language skills. Below here's a diagram from the text that looks at when the accepted range of 'normal' happens for particular sounds, and some ideas about language.
Speech sound production.....
Here's some ideas about language development...