I am so sorry. I had to go through that with my cat a couple of years ago. She was also 17 years old and dh and I took her in as she was a stray a few months after we were married. My cat, Sparkles, had not been sick though. Just one Saturday morning something happened to her. I thought it would be cruel to start putting her through a lot of testing and poking, prodding and all since she was so old. It was the hardest thing I've ever done thus far in life. I still miss her like crazy.
I know you'll get through this and your kids will too but I know it's also so very hard. Many hugs and good thoughts to you and your family today!