Seuss, Dr. (1999). The Lorax. New York: Random House.

I enjoyed this story. I think it presents a compelling message that resonates even today, and it is done in such a way that children readily understand the point he is trying to make. It is didactic in the sense that it is intended to teach children what happens when greed overpowers good sense and people use all of the natural resources without thought to the future. I think there are didactic books today. Books, especially fiction books, are a wonderful way to teach children lessons in a fun way, so that they do not realize they are learning. A few examples are:
Hoobler, Dorothy and Tom. (2002). Earthsong. Brookfield, Conn.: The Millbrook Press.
Child, Lauren. (2001). What planet are you from, Clarice Bean? Cambridge, MASS.: Candlewick Press.
Cherry, Lynne. (1990). The great kapok tree. Orlando, FL: Harcourt Brace & Company.
Cherry, Lynne. (1992). A river ran wild. Orlando, FL: Harcourt Brace Jovanovich, Publishers.
These are just a few examples of children's books that could be considered didactic. Admittedly they may be as subtle as the Lorax, but they are good books with wonderful messages.
Picture copied from Amazon.com.