Your question: How to I grade my child's work?
Do you yourself enjoy being graded?
One of the glories of homeschooling is that learning can be a more natural thing. The only thing that really counts is whether or not your child has achieved mastery over the material—not how long it took.
I encourage you not to put any grading pressure on your child.
Two Thoughts about Grading
1) For internal use only. If you insist on grading your child, and your child is working in the books that have The Bridge (those books before Life of Fred: Beginning Algebra Expanded Edition), then the requirement listed in the book is that the child must get at least 90% correct at any Bridge in order to proceed any further in the book. By almost anyone's standards, your child is showing mastery of the material and should receive an A.
In the books starting with Life of Fred: Beginning Algebra Expanded Edition, there are three Cities at the end of each chapter. If the student can do any one of the three Cities with only one error, then award an A for that chapter.
If the student's best score on the six Cities is three errors, then a B.
Four errors = C
Five errors = F
2) For transcripts. I taught for years at the high school and at the college level. Grading is, in the final analysis, so subjective. Does there exist, for example, an "objective" measure of one's acting ability? Or of one's compassion for the plight of humanity?
Even when I taught the same math course at the college level during the day and during the evening, I used different standards of grading in the two classes.
Uniform standards for grading simply do not exist. The 90-80-70-60 standard is, in my opinion, hogwash. It depends so much on how hard the questions are. When I was a student a thousand years ago, those teachers that proclaimed they were grading 90-80-70-60 often had to make "adjustments" after the test was given because too many students scored either too high or too low.
I've been told that the average grade today at Stanford University is A-. There has been an extreme inflation in grades over the last 50 years at many schools. The average grade used to be C.
If you are called upon to have to report to external authorities (e.g., on a transcript), the only reasonable grade you should report is A. Honest. I'm not kidding. Grading is not objective. To give a grade of B, is to almost certainly exclude your child from a chance of attending some universities. Why do you think that SAT exams are given? That's the only real way to level the playing field.