I think it would be an excellent idea for the Nanny to have the opportunity to take the kids out on daily outings.....
Possibly to some new places.
Maybe the Nanny is just getting bored doing the same things around the neighborhood.
Perhaps she would like to show them around your local zoo, a different park or playground & an A/C indoor mall for hot days.
I think as long as you make sure that her driving record is good (have her provide your family w/a recent DMV printout), make sure her Driver’s License has not expired and that she is fully insured, this could be fun for the kids!!
You can pay to have her vehicle professionally inspected by a certified auto technician or split the co$t of a used car w/the other share family.
Good luck!