Ask a teacher friend to grade a sample of their work after you. This second opinion might make you aware that you do grade harshly, or provide good evidence to parents that you're not!

I would set up a conference with the parents and another co-worker or the principal. I would go over their childs work with the rubric and start with showing what the student has done correctly. I would also have samples of other students work who completed the whole assignment correctly (not showing their names) so the parents can see what the final product should look like. Maybe give rubrics to your students for their assignments as checklist and remind them to make sure they have done every area that was required.

trying showing your own example of a student who scores at different levels of your rubric so that you can show the differences on how you grade and where the grade lands, you may need to involve your administrator if this persists

You could gather some student work samples along with the rubrics and have a parent conference with an AP or other office staff present to provide support. If you have enough physical evidence that the student is achieving these grades themselves, that is your job done. The rest is up to the parents, and if they see their child's work and still think you are faking something, that's a very serious accusation and one that should be overseen by a school official. Though, they are probably just looking for something to be mad about. It's not your fault at that point.

I would try to set up a parent teacher conference to explain further why they received the grade they did and what they can do to improve it for next time. You can even show others work, not showing the names, to show parents what it's supposed to be like and consist of.

You could get a second opinion and have them share with the parent. You could also get the principal involved to help back you up on this.

Meeting with them face-to-face is what I would do first. Show them their students work and the rubric side by side, and if that doesn't work you may need to get a third party involved. This could be another teacher that can explain what they see in their students work or the principle.

I think having another teacher grade the same assignments at question would be helpful. I think if the parents continue to accuse you of sabotaging their child, administration should be involved as that is inappropriate especially if you have given them rubrics and proof or validation for their child's scores.

If the parent insists that you are not grading their student's work fairly then giving the assignment for another teacher to grade using the same rubric will give them a second opinion.