When is the next time you are scheduled to see your S?
Maybe say, "I don't mind, hopefully you don't mind me having "E" on _______this day."
Now would be the time to throw that in there as you are giving her a positive answer IMO.
Her picking our son up from daycare is just a couple or so hours from my time with him today, so it's not a HUGE amount of time. There will probably a time soon when I will take our son out for a similar occasion.
This is the first time she's ever done it in the middle of the week, though, so I wonder if she'll make this a habit.