Responding further:
What is the name of your state (only U.S. law)? Ohio
Is there a law that says you have to do homework?.
No. There is no law that you have to do homework.
There is also no law requiring that you be given a passing grade.
School districts and individual teachers may have requirements for passing a course, including homework.
School districts and individual teachers may have policies in place that cover instances when homework might be excused or an extension in the due date allowed. For example, if one has a documented illness, one may get an extension. If for religious reasons one cannot do an assignment, on can get an extension or request a reasonable substitute assignment. For example, if your religious observance of a holiday requires no work, including no writing - but generally, this would have to be for an organized religion.
Are kids protected from failing if parents say no to homework? .
No. Kids are not protected from failing if parents say no to homework, unless one of the parents is the teacher.
Of course, if the reason the child is not doing homework has a sound basis, recognized by the school district, and the parents have informed the school district of this reason in an acceptable fashion, an exception is possible.
School work should be done during school hours and not family time.
Homework has the pedagogical purpose of reinforcing what has been covered in class. It can be a useful tool to encourage practicing new skills, further reflection of material, or independent exploration.
Some homework is busy work, especially if one has an intuitive grasp of the material.
Homework can be an opportunity for family to learn together. Learning time management skills and how to prioritize what needs to be done is useful.