FWIW, I can see why you don't want to commit to this for (possibly) the next 2 years, but I did think you should do it, albeit with a few ground rules.
For eg:
•If you're ill, then you can't really ask whoever takes your dd to also take another child.
•If your dd isn't going to school then you're unable to take May.
•If she's late, you can't wait and will have to assume she's not coming and so you will leave at your usual time.
•If you have to leave early for work then you're unable to take May unless the mum's happy for her to go to breakfast club at her own cost, and can have her ready early enough so that you're not late.
I think if you set some ground rules, it would benefit both parties. You could always agree to review things quarterly, etc, so that if anything changes it can be dealt with.
I think because you're not having to go out of your way (& they're nice people), etc, it would be a nice thing to help out.
With ground rules in place, I think it'll teach your child about helping others (but not by being a doormat), and it's the right thing to do, morally speaking.
Whatever you decide though, I'm sure she's grateful for the help you've already given, and will understand why you can't commit on a more permanent basis.
Fwiw, I have rheumatoid arthritis, fibromyalgia and various other conditions, and I'm definitely worse in the mornings.
I'm just lucky that my dc are now old enough to get themselves up and to school without me.
I took them to school myself all the way through nursery/primary school (with occasional help from my mum), including several years of driving them 48 miles to and from school (12 miles each way).
I did it myself most of the time partly because I had no friends to ask, but mostly because I hated asking someone else for help as it makes me feel pretty useless.