Speech and Language UK's website is a treasure trove of resources to understand aspects of speech and language development. 

Home - Speech and Language UK: Changing young lives

Their guidance document, Universally Speaking, provides detailed expectations for speaking and listening at different ages and can be used to develop understanding of progression in spoken language. Teachers can download this document for free:

Universally Speaking 11-18

While the above document is pitched at the secondary age range, it can be useful to understand the developmental stages before secondary school in order to support students who struggle with speech, language and communication.

Universally Speaking 5-11