I know that there are times when one of my parents is not working while I'm caring for her child. Should I subtract payment for those days when I give her a receipt at the end of the year?

No. Even though parents are only entitled to claim their child care tax credit if they are working or attending school, it's not your job to police this. You may not have all of the facts. Give her a receipt for the entire amount she paid you for the year.