I need a little help

I received an email from my employer today (small business, less than 20 staff) - Lovely lady, in her mid 60's - DH also works for this company. My employers grand daughter is our childrens baby sitter. We regularly have her on a Monday after school, to sit the children. This is not a love job, it's a paid job.
Anyway, one of the paragraphs in the email stated:
"Would you be able to work later on Tuesday?? Do you want to see if H is free?"
Now, rightly or wrongly, this has gotten my back up a little.
The baby sitting is an agreement between H and us - not our employer, despite her being the grandmother.
Secondly, by her "offering" H's services, she is assuming that I can work should my kids be taken care of, and that I can afford to pay her.
The simple fact is, I cannot afford to pay the sitter for those extra hours (my hours will be juggled, so I wont be working/paid for extra hours) - but I need a tactful way to advise her of this, and also advise her that the care arrangements of my children, and My availability to work is my responsibility, not hers.
Can anyone offer any suggestions?