We are born of that innocence, not a care in the world (save perhaps for where the next meal is coming from), no judgments, no hatred, no resentment. So when does it end, and why? More to the point, how do we restore it?
The innocence ends when we take our first bite of the fruit of the biblical tree of knowledge of good and evil. The moment we pronounce ourselves the judges of all things, when we pretend that we know what is good and bad for others, what is right and wrong with the world, how others should behave and what God should be doing to support us, we lose our innocence. That is the moment we start pretending to be God, trying to direct how the world and our lives should be, as if we were God. Innocence is gone and we are simply left judging, hating, resenting, demanding and pretending.
How do we recover the innocence? Stop pretending to be God. Rather get real about who we truly are and accede to the way that Life is, not how we demand it should be. It's that easy (in theory at least). The only challenge is to train our minds to stay put in that innocent state as we go about our daily lives.
Which is not to say that we are required to stay passive and do nothing, allowing Life to have its way with us. We can still have big purposes for our lives and live our lives in order to realise those purposes. If the purposes are wrong, Life will always let us know, but we can still live with purpose and recover the innocence.