In the early stages of language development, children goes through milestones such as
Use of verbs
full sentence etc. [I may be limited in my knowledge so correct me if so]
Now by the age of 2.5 to 3 (and in case of my kid it is at 3.5 years) children do express themselves reasonably well.
However, there are many kids at four which are very clear in their expressions just like any other adult and are also comfortable expressing ideas at a bit more complex and abstract levels; where as in my kid (also observed in some kids) manage to speak only actionable (often not very fluent) or in other words their ideas are restricted.
My question is: what are the milestones of language (and if you consider it as cognitive) development of children after the are able to get basic speech? How do they evolve from there both in developing their languages and their understanding of details of the world?
PLEASE NOTE: i don't know if cognition and language are two independent things. I, as a parent, only want to be knowledgeable about a typical growth path that children follow (or are expected to follow) after basic speech is achieved.