Honestly, whenever I felt like I had too much homework, I just didn't do it. You can always discuss these things with the teachers. Usually they're not complete dicks that want to ruin your life.
Let your kid grow a spine. (continuous) Sleep loss as a kid is very bad.
When I didn't want to keep doing homework in High School, they dropped me from "honors" math to regular math and gave me poor grades. They continued to give me bad grades occasionally (since I didn't do the homework, occasionally) but it seemed like everyone met a decent compromise and the teacher didn't have any particular ire towards me.
Let your kid grow a spine. (continuous) Sleep loss as a kid is very bad.