I'm so sorry, 4U. What devastating news.
From someone who has had to put down my last two service dogs from cancer, i hope you won't mind my sharing that now is the time to make special memories and take special pictures. The kids with your dog. You with your dog. Let those tender moments be caught on film.
I know it's very had to even think about doing this, but down the road, you will appreciate having those pictures so much. Take a paw print (poster colors on paper) and cut a small lock of hair. These are keepsakes that may mean so much to the children down the road. And, when no one's looking to see a grown man cry, for you, too.
I have hundreds of pictures of my service dog Peek, but by far, the most precious to me are those taken in the last week of his life, that show his love for my husband and I shining through. Though he lacked energy, and was mostly sleeping, those moments captured when he was awake, and looking at one of us--are the most precious of all my possessions.
My heart is with you at this oh-so-difficult time.