I'm sorry for your loss, OP.
As a parent with a minded child, I would have been concerned about leaving her with you the first week, and a new assistant that no-one knows wouldn't necessarily done much to alleviate that feeling.
It's a nightmare when a Childminder is sick - and I appreciate immensely how much effort you put into not letting people down when you would, quite rightly, have been entitled to just shut down for a few days - but I would have been worried about the standard of care.
If you are sure that that standard of care did not slip during those weeks - and I'm not saying it did! - then charge them for both weeks, as per your contract, because it was only their choice to go elsewhere, as it would have been mine.
I would have sought other arrangements, but I would have appreciated that that was my nerves (and my need not to be an arse with someone who's grieving!) and would have paid you anyway.
Certainly, by week two, they're out of line, and should pay, or surrender the place.