A.M.
I hope that's a rhetorical question about whether or not you should do something. You said in the last post she has raised them for 6 yrs. since they were newborns and that she was pretty much faultless.
YES! You should do something, not only for her sake, but also for the sake of your children. Think of the message it would give to them of the insignificance of those who care for them and who they care about if you did nothing.
Whatever you do should probably at the least include you NOW taking a very nice photo of her and children and giving it to her framed. The children could write her letters or make a list or "poem" about their favorite things, qualities, activities with Nanny. They could draw pictures for her. Put it all on a poster or make it nice and frame all that too or make it into a book like on Shutterfly titled What We Love about "Nanny" or 10 Things We Love About "Nanny".
Maybe a charm bracelet with charms on it that the children pick out that reflect their 6 yrs together or their favorite activities together or Nanny's qualities.