Heather Shumaker, writing for Salon: 

Homework has benefits, but its benefits are age dependent.

For elementary-aged children, research suggests that studying in class gets superior learning results, while extra schoolwork at home is just . . . extra work. Even in middle school, the relationship between homework and academic success is minimal at best. By the time kids reach high school, homework provides academic benefit, but only in moderation. More than two hours per night is the limit. After that amount, the benefits taper off. ‘The research is very clear,’ agrees Etta Kralovec, education professor at the University of Arizona. ‘There’s no benefit at the elementary school level.’

Let the kids play, be free and break the cycle: 

Then there’s the damage to personal relationships. In thousands of homes across the country, families battle over homework nightly. Parents nag and cajole. Overtired children protest and cry. Instead of connecting and supporting each other at the end of the day, too many families find themselves locked in the “did you do your homework?” cycle.