When I was a SAHM and babysat at my home, I had a contract drawn up.
I will get paid for this many hours regardless. You will drop them off between this time and this time and you will pick them up no later than this time. I also put a clause in there that stipulated late fees as well. (anything over 15 minutes was $2 a minute. And I made sure their watches were in sync with mine).
I watched 5 kids in my home not including my 2 older children. It became a habit for one of my parents to either drop off unexpectantly or pick up WAY late, her children...and it seemed that I got them on days when she just wanted to "sleep a while" too.
The contract saved all of us a HUGE headache with miscommunication. And it also spelled out that I would have a vacation and not be available for a couple of weeks out of the year as well.
Smiles to you. Good Luck!