I won't tell you what you should do.
You should know though, that if someone is using you and you feel like they're taking advantage of you, and you accept it and put up with it and don't say anything, you're not a victim, you're a volunteer.
Regardless of the health of Grandma, you can create reasonable boundaries for yourself and then maintain them, if that would give you more peace. If you aren't willing to do that because you'd rather be used, then your best bet is to enjoy being of service and stop being resentful. This woman hasn't harmed you. She has only asked you to do things for her. You never had to do any of it. You can set the terms of your relationship. No one else can do it for you.
I know this sounds a little harsh, but if you think you are a victim, there's no hope for you. If you can claim responsibility for your behaviors that got you into this uncomfortable place, you can change what you are doing and thereby change your situation. You cannot hope to change your neighbor, but you can change yourself. Decide what you want your life to look like, decide what friends you want to keep, decide what people you don't want to spend time with, and make it so.
Or leave things the way they are and decide to like it so you don't have to feel resentful.
If you really don't want to watch her daughter as they wait for the bus, tell her immediately. It's Christmas, there isn't school anymore for at least a week. That's plenty of time to make other arrangements. If you don't tell her, then clearly there is some reason you WANT to watch her daughter. If you can figure out what that is, it might be helpful to you.
I wish you well.