My youngest, who has had au pairs all her life, was hit very hard when one of her au pairs left -- DD was 7 at that time, and this au pair had also really formed a good bond. It was hard -- no ways about it.
One of our au pars created a posted with pictures of the tow of them, which dd LOVED. So maybe have dd and au pair prepare a scrap book, along with special messages from the au pair written in it, that she can keep and look at.
And yes, skyping helps a lot! This au pair also had included dd in skypes with her family back in Poland, which helped form a connection to the family back home.