At the start of this piece, you made a comment about homework being "necessary." This really depends on what you mean by the word necessary -- necessary to learn or necessary to pass. The sad truth is that research gives very little support for the notion that homework helps in learning. The problem for most parents is that homework is factored in the grading system so heavily that it becomes necessary to do it to pass the course.
If we think about it, we realize that homework is given an extremely high weight for the time it is supposed to take. The child can earn a zero for work not handed in, He may score 60 on a test he failed, yet the zero has a 2 1/2 times greater, negative effect on the grade than the failing test had. And the homework score may get factored in 20 or 25 percent even though it should, theoretically take less than 10 percent of the child's total school/home educational day.
When you talk about having your 9 year old child do 30 minute blocks of work, I would say that a 9 year old should not be doing more than 30 minutes work in total. So my recommendation to you is to limit yourself and your child to one such block. Bring the work session to a close, once the 30 minutes are over, and live with the results. Your child will probably do more in that time bound 30 minutes than he will do with the current arrangement.
This obviously puts you in a position of having to deal with the school around the work that has not been done, but you are dealing with them now, while homework dominates your home.
Good luck. For more on this model, check out www.thehomeworktrap.com.
For more information on Dr. Goldberg's model, read other postings on this blog, visit his website, The Homework Trap, or read his book, The Homework Trap: How to Save the Sanity of Parents, Students and Teachers.