OP . You are far too polite.. Been there. Done that.
Stop expecting your friend to be about to ask you something fun or wonderful.
She's constantly just asking you for free child care.
as a pp said, she's leading with the typical CF question. She's learnt that this is a good way to trick you into saying yes.. because she doesn't believe your time is as valuable as hers. And she thinks that because you yourself don't really believe your time is that valuable.. and you've been trained to think you should always do others a good turn if you can. (which is admirable with people who deserve it, but does lead to being taken advantage of by unscrupulous people who don't see it as a virtue but as a weakness and so they are often not even grateful but see you as "useful" to them. )
So the next few times she asks this just say.
Yes I am busy. and wait, wait, wait, wait
Until she spills and then say flatly "no I can't do that" (with no window dressing)
"Why?" - "Because I don't want to."
If you start giving excuses she will start picking them apart until you cave. I found saying " I can't believe you want an actual list of what I will be busy at." was very effective.
I think you could even try saying "I was just thinking how nice it would be if you were asking me that to invite me to something exciting, but turns out its always just asking me to look after your children." (Merry laugh)
You will feel guilty and rude for saying no. But you just have to do it.
She's not feeling guilty or rude for constantly asking you to look after her DC is she? And yet you wrote this thread because it was beginning to make you feel used.
You have the right to say NO. Start exercising it. Be proud of being able to do this.
I think saying no a few times will stop you being on the top of her go to list for favours and it sounds like you've already done more than enough. And then you can say the odd yes when the occasion really warrants it. Good Luck x