I'm in an awkward situation.
I provide childcare. For the most part, I adore the child I watch. Let's call him Bob. He and Little Knit get along amazingly well. They are as much BFFs as toddlers can be. She calls him "hers" (ie, Dat's my Bob). Bob's mother, Jan, and I are pretty good friends, too.
I've done childcare in the past. In the past, it's always been a set schedule (ie, Monday, Wednesday, Friday). I have liked this because I can set my schedule for the week and know what days I'm working and not.
(edited to clarify: my current schedule is not mon/wed/fri, but is more 'random') Jan works shift work, and the nature of her work is such that she gets her schedule 6 months out so she can make changes as needed. This initially looked like it was going to be great for me, because it also means I get my schedule set 6 months out.
Understanding that the nature of her work is such that she'll want the freedom to change shifts, etc, I only write down my schedule 2 months out.
Except: She has a terrible habit of changing her schedule for next week and then dropping it on me.
She'll email me a week in advance to remind me of my schedule for the upcoming week (unnecessary, but I can appreciate her need to confirm). But when she does this, she'll often ask to change something or other. Last weekend, she emailed me to ask about a change in the schedule for this week. It was not a big deal, so I agreed. Yesterday, she emailed me again and asked to make another change. This change is because she made a shift change at work. This one kind of IS a big deal. I had several things going on that day that I now must either not do or must reschedule. I cannot take Bob with me for these errands for just a whole lot of reasons. I can reschedule some of the errands. But the day she wants me to take Bob is actually the most convenient day for me to have the one vehicle Mr K and I have.
I can do it, but I don't want to, it's inconvenient, and I hate that she changed her schedule without consulting me to see if I was even available.

She doesn't have much in the way of backup childcare.
Please help me figure out a way to communicate to her that her schedule changes affect me.
We addressed just about everything else before I started watching Bob, including what to do if I am sick, he is sick, Little Knit is sick, vacation time, overtime, overnights, meals, general expenses of things such as milk, diapers, wipes, etc, early dropoff, etc. This was never discussed because short-notice schedule changes never occurred to me.
Now I feel awkward about it and am not sure how to go about handling it politely - especially because she's also a friend.