How we cite our quotes:
Who can recall to me the sins I committed as a baby? For in your sight no man is free from sin, not even a child who has lived only one day on earth. (I.7.2)
We tend to think of babies as innocent, but according to Augustine, ignorance does not equal impunity. As Augustine points out later in the paragraph, the fact that it definitely would not be acceptable for an adult to throw a tantrum shows that tantrums must be sins for children, too. The bigger point here is that no human on earth can escape sin, which means that everyone, no matter how good, needs to seek out God's mercy.
But my sin was this, that I looked for pleasure, beauty, and truth not in him but in myself and his other creatures, and the search led me instead to pain, confusion, and error. (I.20.1)
This is a pretty good summation of sin in general—at least, sin of the non-violent variety—and it comes at the tail-end of the chapter on Augustine's childhood. This is right as he's entering into the world of adolescence, where much more serious sins are waiting for him. Rather than saying "I liked sex" or "I liked praise," Augustine instead tells us that his real problem was that he cared too much about himself instead of God… and that opened a whole can of issues for him. Kind of like Pandora's Box.
Let my heart now tell you what prompted me to do wrong for no purpose, and why it was only my own love of mischief that made me do it. (II.4.2)
You know the trope of the Justified Criminal? You know, a character who does something bad but for a good purpose and you side with him? Augustine is not that character. His shenanigans are purely for kicks and giggles. When he sits down to try to analyze his behavior later in life, he can't even really come to any conclusion about why he did what he did. Except that, without God in their lives, people flounder around and do all sorts of inexplicable bad things.