Oh gosh, I am so sorry.  She is gorgeous and I know this will be difficult for you all. 

It is never easy, and of course, it's always hard for us to discuss these things with very young children - but you know, children are a lot more accepting than you think, sometimes.  They have a limited awareness of what death means, and it can be surprisingly easy to help them through something like this if you approach it without too much expectation.

I'm sure the books linked to will have better suggestions than I will, but I'd just explain to her that Ursa is very sick and though everyone thought she was getting better, she is not, and that very soon her body will be too tired to continue.  Gently tell her that nobody can help Ursa, and when she is too sick to enjoy life, it's going to be her time to leave you and move on to wherever your belief system says we go after we die. 

If you don't have a strong faith or belief in a hereafter, you can just say that our pets go to a place called the Rainbow Bridge where they can play all day, or even simply explain that every living thing dies in the end and that we have to leave this earth.  Whatever you feel is acceptable. 

There are a number of wonderful stories circulating on the internet about parents worriedly telling their children that a beloved pet must die soon and the child coming out with something so wise and spontaneous that it takes their breath away.  Children can amaze us.  And we worry so much about them, don't we?  Hugs to you, too.

She's a beauty!  Just a stunner.  Damn that rotten cancer for hitting such a loved, lovely lady.  I'm so sad and so angry for you.  For all of us.  Like Jerry, I'd like to know how it's been assessed she's got "weeks."  It may be more!  Yes?  No?  I guess in the end, it doesn't matter.  Love her up like you always have and enjoy all the times together.  Every day is a gift.

I lost my first family at a young age - not a preschooler, but still young.  It still will yank up some tears.  That's not a bad thing.  Your little biped-human may take it hard, or not.  Maybe since you seem concerned, she's the sort to take it hard.  Who knows, out of this may spring the next great canine cancer Vet!  I love the words that Jay had for her: your pup is sick and will one day be too tired, too ill to fight and there's this wonderful things we can do for our loved, sick, ill pups....

Good luck to your pup and to your little girl!

~ Katy