When I looked at my children and tried to find the words to tell them what had happened, I found myself reaching for the old cliché, "Maggie went to the farm where older dogs like to play". I really believe in telling my kids the truth, so those words never actually came out of mouth. But, I would have found them comforting and wanted to find some comfort for the kids.
To my surprise, my 8-year-old had the most specific questions about what exactly happened at the animal hospital. I was not prepared. I softened the details and simply said that it looked to me as if Maggie fell asleep and the veterinarian told me she had passed. I woke up the next day and wondered if I had not really respected her need to know details, but they just seem so harsh.
We cried together and put a big picture of Maggie on the kitchen table. Like most of parenting, I find that in times like this, I'm "winging it". You never really know the right thing to say or the best way to help the kids deal with difficult situations.
I know experts have written extensively on the subject. You might want to be a bit more prepared than I was, and read some suggestions now!
http://www.parents.com/parenting/pets/kids/pet-death-books-help-kids-cope/
http://children.webmd.com/features/what-say-when-pet-dies
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