I am in exactly the same situation you are! I watch my best friend's little boy 3-5 days a week. I am paid, quite well, considering it's *usually* only 5-6 hours a day, not all day long (she pays me what I used to charge full timers at my daycare, and she and her husband drive out of their way to bring him to my home). What gets me is this almost constant thing of her husband will ask if he can drop baby off an hour early. Then, Best Friend, not knowing her DH dropped off early, will come a half hour or so late because of an appoinment, or running late, etc. When I tell her her DH dropped off early, she is very sorry, and embarrassed. But they need to get their act together, and I pipe up when I feel things are too much, or there is a conflict with my schedule because I wasn't expecting the extra time that day. If she is your good friend, don't be afraid to say when too much is too much. I also ask to be paid an extra $10 per day when they need me to watch him all day- more than 8 hours anyway. If she doesn't know how you feel, (and of course be cordial and nice) then she isn't going to have a clue how you are feeling. And trust me, it's a struggle for me sometimes, but my DH just encourages me to be honest with her and so far, it's worked. We made this arrangement cause they do NOT want their child in a big daycare and they had a serious scare with a previous nanny. So in their eyes, I am doing them a BIG favor, cause I don't really NEED the money- I'm sure your friend deep down feels the same way- she would much rather have her good friend watching her kids than someone she doesn't know.
And, I would tell her, "No it doesn't work for me to drive ALL the kids downtown to meet the dentist...if it's an absolute emergency, I will do it this time, but next time you need to schedule it on your day off or around your schedule."