You sound like a very considerate employer OP in that you truly respect your nanny's time and value her services and I commend you for that because, trust me...Not all families think the way you do.
That being said, what occurred was a one-time thing and totally out of your control. I am sure she understands this and appreciated the fact that you texted her to let her know. And good for you for remembering to compensate her for the extra hour.
If you still feel terrible about it, what I suggest you do to make things right again (even though I am quite sure your nanny is already over it), is in the future, if possible, let her off earlier than planned but still pay her for it.
In other words, offer her some paid time off. That should make you feel a lot better.
