When Enlil was young, he had a baby with the goddess Ninlil, and the other gods threw him out of heaven and he had to go live in the underworld. But Ninlil chose to go with Enlil, and after a while Enlil and Ninlil went back to heaven together.
After this Enlil ruled the other gods, and was the father of many gods. The Sumerians said that Enlil created people, but then Enlil got tired of listening to the people's noise (compare the story of the Enuma Elish, or the Tower of Babel). So, as told in the Epic of Gilgamesh, Enlil sent a big flood to kill all the people (compare the story of Noah). The god Ea helped one man, Utnapishtim, survive the flood, and Utnapishtim repopulated the earth and then became a god himself.
But by around 1700 BC, the Babylonians were ruling Mesopotamia, and they had their own ideas about the origins of the gods. The Babylonians said that their god Marduk was the king of the gods, and that Marduk had defeated Enlil.
Ancient Mesopotamians, by Elena Gambino (2000). For kids, retellings of Mesopotamian stories and lots of context.
Gods, Goddesses, and Monsters: An Encyclopedia of World Mythology, by Sheila Keenan (2000). Easy reading.
Religion in Ancient Mesopotamia, by Jean Bottero (2001).
God Against the Gods: The History of the War Between Monotheism and Polytheism, by Jonathan Kirsch (2004). From Akhenaten in Egypt, through Judaism and the rise of Christianity. Lively, popular writing.
A World Full of Gods: The Strange Triumph of Christianity, by Keith Hopkins (2001). Entertaining account of what it was really like at non-Christian and early Christian religious events. Not for young kids.