Decades of investigation have failed to turn up any evidence that homework is beneficial for students in elementary school. Even if you regard standardized test results as a useful measure (*and in public schools, it certainly isn't; anybody can memorize) homework isn't even correlated with higher scores at these ages.
The only effect that does show up is more negative attitudes on the part of the students. . .
Teach your children to love learning! It's so unfair that out-schooled kids are away from their parents all day and then spending the little bit of time they have with their parents (assuming they don't do any after-school sports or activities) slaving over homework. Schools are so terribly unproductive.
*All italicized words are my own. This quote taken from Alfie Kohn's article, "The Truth About Homework", Education Week, Sept. 6, 2006