Adults are afraid of death and avoid the subject when it is a natural part of life. Explain in words they will understand that Kitty is very old and soon it will be time to let him go to rest, forever. Note the difference between LET and PUT. You are LETTING your Kitty find peace, NOT PUTTING him away. I dislike the term PUT your animal to sleep. It is not a nice concept. Heaven is a very comforting idea and might give your girls some sense of comfort, but you have to be careful, because I know of some children that missed their departed one so much they wanted to go to heaven to be with them. So if you mention heaven at all, you must explain that is where the animal go to WAIT for us to join them. Whatever you do don't skim over the subject of death. The death of a pet is a good time to learn about dying and to prepare for a human death, which eventually they will have to face.
Here is a nice poem.
Rainbow Bridge
Just this side of heaven is a place called Rainbow Bridge.
When an animal dies that has been especially close to someone here, that pet goes to Rainbow Bridge.
There are meadows and hills for all of our special friends so they can run and play together.
There is plenty of food, water and sunshine, and our friends are warm and comfortable.
All the animals who had been ill and old are restored to health and vigor; those who were hurt or maimed are made whole and strong again, just as we remember them in our dreams of days and times gone by.
The animals are happy and content, except for one small thing; they each miss someone very special to them, who had to be left behind.
They all run and play together, but the day comes when one suddenly stops and looks into the distance. His bright eyes are intent; His eager body quivers. Suddenly he begins to run from the group, flying over the green grass, his legs carrying him faster and faster.
You have been spotted, and when you and your special friend finally meet, you cling together in joyous reunion, never to be parted again. The happy kisses rain upon your face; your hands again caress the beloved head, and you look once more into the trusting eyes of your pet, so long gone from your life but never absent from your heart.
Then you cross Rainbow Bridge together....
Author unknown...