What is the nativist approach in terms of Child Language Acquisition?

The nativist approach was put forward by Noam Chomsky, stating that children's brains contain a Language Acquisition Device which holds the grammatical universals.

This theory came about as children have been observed to pick up grammar and syntax without any formal teaching (in spoken language). They seem to learn these fundamentals of their native language(s) purely from the input around them. 

Chomsky believes that the LAD helps children decipher the grammatical structures of their native language(s), subconsciously mapping new lexical items to their corresponding word class and syntactic position. The LAD could in theory mean that children while possessing this part of the brain could easily pick up the grammatical structures of any input language as they already have the building blocks in their mind.

This theory is contested by a lot of linguists due to the fact an LAD has never been found on brain imaging or in other studies of children's brains. There are many other approaches which contradict Chomsky's theory but the nativist approach is still widely held in high regard by many language development experts. 

The nativist approach in no way suggests that children are born with a lexicon, the majority if not all linguists agree that lexical items are learned from input and social environment. The different approaches to language development mainly focus on how children learn grammar and syntax.