Recently my son (who is in upper primary) was reading 2 Chronicles. As it happened, I finished reading 2 Chronicles not long ago. I said to him that I thought a message of the book is that we should rely on God rather than ourselves or other resources. (The successive kings of Judah - as recorded in 2 Chronicles - had to learn to rely on Yahweh. It's a point clearly made by the chronicler.)
Then my son said to me, "I think the message of 2 Chronicles is that people do sin against God, but God still forgives them." Then he said that the kings in the book kept sinning, but God is merciful.
I think he has a point, and in fact spot-on.