@Sheril Bluntly at kindest, harshly at worst. More likely it would be something like this:
"Dear so-and-so,
I decline to carry out your request, and I doubt any of my colleagues will be willing to carry it out either. I suggest you not contact anyone else regarding your desire to be academically dishonest on behalf of your child.
Sincerely,
[your name]
P.S. Do not contact me again on this matter.
"