This is how cheeky fuckers work - they rely on you being too polite to say an outright no and they always have an answer to whatever white lie you tell them!
Honestly the best approach is to be direct and say you don't want to share school runs or do their after school childcare. They aren't worrying about what you think, they aren't your lifelong friends, so why are you turning yourself into a human pretzel, to avoid saying no?
I be had 4 kids go through primary and not one had the same friends in secondary that they had aged 4. Parents get all caught up in fostering these friendships, as if they are all important and will see their child through their whole education, but it's just not true. Children's friendships are very fluid at this age and there is very little chance of you knowing this woman forever. So why the reluctance to offend?
You need to be clear and unequivocal. I always used to say in the presence of a cheeky fucker that I liked taking my own child to school, so didn't need a reciprocal arrangement and I wanted to give my child one to one attention and not look after anyone else's child during that time. This is something they can't really argue with because it isn't a weak arse excuse, it's an actual reason.
Obviously I helped our parents I knew and liked with emergency help but I wasn't ever willing to be anyone's default childcare. You have to weigh up what's in it for you and the answer is nothing.
From what you've said, it would actually benefit your own child to create some distance - she's looking to you to learn boundaries, so you have to show her that you know how to maintain yours!