The Lamb, and the Tyger are two different poems, therefore the tone in each is a bit different. The tone of The Lamb is that of innocence and a reverse of the fall from grace by mankind. Childlike in nature, the tone is that of a perfected human nature.
The Tyger's tone is that of reality. The Tyger is not beyond redemption, but has a darkness to him that tempts him into wrong choices.