I have been thinking about John almost every day since his troubles began, possibly because he is a twin to our Havanese, Butter. Your familiy obviously gave John a wonderful and long life. Then you were there to help him on his journey when he most needed you. This is the last thing we can do for a beloved pet after all the years of unconditional love they offer us. How wonderful that you were there for him at the end of one journey and for his passage. I do believe in the Rainbow Bridge.
When we lost our big Maine coon cat, Cecil two Christmases ago, I was so sad and frightened about the inevitability of that decision. It was a first for both Marty and me. We took care of Cecil for about three months and knew when his time came. Wish that I had known ahead of time how very humane and peaceful this kind of end can be. Our vet came to the house, gave Cecil a shot of Valium and he just went to sleep before the second shot. All very undramatic throughout and I was so happy that we could hold and stroke him until the very end. This is a responsibility we all take on for our pets from the day they come into our lives. And you were right to not try and protect your children from the reality of it all.
My heart goes out to you. We placed Cecil's ashes under our Christmas tree that year, since he left us only a week before. We did the same with our previous cat, George some years prior. I will always remember Cecil going under the tree and sleeping with his head on George's little pine box under the tree. He did this every year. And the funny story was that after George died, Cecil looked everywhere for him and (we think) decided that George had disappeared into the VCR slot. Cecil would paw away at the slot and meow piteously until we opened it up for him to take a sniff! (George used to hide in that cabinet to get away from the more playful Cecil, you see.)
The thing that helped me most after losing Cecil, my constant companion for some 14 years, was making a scrapbook of his life. It included photos from kittenhood up to his last days, locks of beautiful strawberry blonde hair, some pieces from favorite toys, and a print of one of his paws, his collar charms, etc. Not only did it remind me of how vibrant Cecil was for most of his life, but the comparison of early and latest photos reminded me always that we did the right thing for him at the right time. That was very comforting. You are in our thoughts and prayers.