Awww this is not an easy subject
Just before christmas we lost a 10month old cat to a road traffic accident. I remember it as if it where yesterday
The bad thing was I had my dd with me, so it was awful.
I saw a black cat lying on the grass verge, and you know that feeling, when you "just know". I got out of the car (he was lying just round the corner from our house) and walked over. He was in a bit of a state, but whoever found him, was kind enough to lie him grusome side down. Fortunately I had a blanket in the car, so wrapped him up and put him in the boot.
My DD although only 4, seemed to understand bless her. She said whats wrong mummy is that giz? I had to choke back my feelings but said that Giz had been in an accident and has gone to sleep for ever. I could see her thinking about what I said, and while i phoned the vets to arrange a cremation I obviously became very upset. She had a few tears too, but bless her heart, gave me a huge cuddle.
I ran her to nursery and took giz to the vets.
The worst bit was explaining to my DD that giz would not come back from the vets (I have him in a casket by the fire place with his ashes)
She kept asking when he would come back, and I did find this very heartbreaking. She got very upset that night, but with a flash of inspiration, I said that Gizmo has gone to the stars to live forever, and that he is chasing mice on the moon, as when cats die thats what happens. But I bet he is having a great time.
It was better when we had the casket back, she seemed to accept it then in a strange way. She still mentions him now and again when she sees the moon. This always brings a tear to my eye
It is hard, but I found explaining it in a way that she understands helped, iysyim. Obviously with your dd being 6, she knows more. I am not sure I would want my dd to see any pet being put to sleep, until they were old enough to want to and know what it means. But then again, it may help with the grieving process.
Have you thought about cremation or anything? I have mine in caskets (3) and they are by the fire. Unless I scatter the ashes myself, I want them buried with me, when its my turn. Sounds a little daft and morbid, but I do love my boys
Maybe you could have a special place for your cat where your dd could go, or a little plaque made. Just a few suggestions. Speak to your vet too, see if they have any suggestions