Could you not offer to do two days a week? That way you won't feel mean for saying no, and I am sure your DD will understand that you don't want or need to be tied down to fetching the children five days a week.
I know it can be difficult for working parents, but surely, she and another mother or father could share the school runs.
Is it quite impossible for the children to come home from school alone? I don't know how old they are, what kind of distances are involved, but a lot of parents are fetching and carrying children instead of letting them learn to fend for themselves. Obviously if the are five they can't go home alone, but if they are 11 that's a rather different kettle of fish.