In Memory of Ruby

Today is a very sad day in our family because Ruby, our very old cat, passed away. Ruby was about 18 years old and has had a long life and lived in many places. Matthew said "he bought her at the pet shop because she was the sorriest looking little kitten in the corner, and no one was paying attention to her". She was a sweet cat and was terrific with the children.
In the past week Ruby was quickly losing weight, not eating much, and not acting like herself. And then in the past few days, she quickly went down hill; she was not eating, had a hard time walking, crying a lot and just looked miserable. So, Matthew brought her to the doctor's office on Tuesday morning and they gave her fluids and medications to keep her comfortable. They did some blood tests to rule out anything that could be minor, but today they said she was dying of old age. We will miss her terribly. What do we say to our children? I don't know what to say to them without crying and upsetting them. I know we need to say something, but I'm not sure when or what to say. At least Ruby is not in pain anymore and is in a better place. We love you Ruby!!
Love, the Mead Family