Hi A.,
I am a mother of 3 children and when my children were of childcare age my grandmothers (paternal and maternal) baby sat for my husband and I.
My father's mom did not take a dime from us (she and my grandfather are pretty financially stable so she didn't feel the need to take money from us) because she always told my husband and I to save our money so we could take care of our family, but on paydays we would treat them to dinner to show them that we appreciated what they were doing.
On the other hand we did pay my mom's mother to babysit because her financial situation was a little different. We certainly didn't mind giving her something, because what we paid her was not what we would have had to pay someone at a daycare facility.
When I was growing up my mother paid her mother (my grandmother) to take care of my sister and I and she didn't have a problem paying her mother either.
You do deserve some kind of compensation especially if your son is in a position to pay you something and if you are running a business your son should understand that there are expenses that come along with running a business.