My children were older, 10 and 8, but I was advised to say, under different circumstances though, that he went to Heaven because it was his time, and that it is ok to be sad and Mommy and Daddy are very sad as is our family too, but that it does not change our love for him, or his for all, and whenever we feel sad we should talk about it and remember something happy about them. For me, I simply said that he had an accident and went to Heaven. I was advised not to be specific as the grief is enough to start, and they will let you know when they are ready if ever, to hear anything specific. I did not want them carrying anything but the loss and then healing in their hearts. If they are playmates of Miss Nancy's children, I would reassure them also that you are not going anywhere, that it is rare to lose a Mommy, and to try to be strong for the girls as they will need their pals.
Now, Im gonna go be sick, really sorry for your loss.