One of the maxims of universal grammar is that children's language acquisition indicates the existence of a genetically preprogrammed language faculty. Because a child cannot master certain complex tasks, yet manages language - so the argument seems to go - there must be something else, something innate that enables said child to speak and understand sentences from an early age.
This seems a fairly convoluted conclusion. Considering that millions of children acquire languages every year, and that a sizeable subset of these children learns multiple languages with great ease, the more fitting conclusion seems to be: "language is not too complex for children to acquire". (For, at the risk of sounding painfully obvious, they are children and they do indeed acquire it.)
The nativist deduction seems a non sequitur.
What exactly is it about language that should be too complex for children? How would one define this complexity? Are there any neurological reasons why this complexity should not be grasped by a child's growing brain?