OP - I appreciate that you care about your nanny.
I see SO many posts on these boards where the nanny is not treated as though they are a human being with a family, concerns, needs, etc.
Someone is always asking a question designed to seek approval to save a buck on the back of the nanny or require the nanny to meet expectations that they themselves would not meet.
It's probably a better use of her time to pursue other positions than anything else.
Please let her know that Same as every other position that all of us didn't get hired for many factors are there. She may never know why they withdrew the offer but since she hadn't even started yet, it is not personal. Usually you find out you dodged a bullet when something like this happens.