Innocence has been explored as a concept in video games since storytelling methods became advanced enough to utilize themes. This is not just limited to the ability of youth overcoming evil (Ristar, The Legend of Zelda: Wind Waker, etc.), but the destructive force created by the loss of innocence. Secret of Mana, Panzer Dragoon, Astal, and dozens of other titles have shown that capable hands can use the medium to express the power and sadness inherent in the transition from childhood to adolescence.
Ico takes this to another level entirely. Not simply content to utilize age-old motifs and themes (burgeoning sexuality as seen in the character of Yorda, the dark magic inherent in child sacrifice), it plays on the themes of simplicity with regards to gaming itself. This is a game simple enough for a child to play, to understand, to interact with… but one that does not reveal its darkness in full until the gamer has already become enmeshed in the story and mechanics.