T.F.
We've been through this starting when our dd was 5 (she is now 13) . We were truthful that Maggie was very sick and the Dr. did his best to save her but could not. We have her ashes on the mantle.
When Frankie died, dd was much older and it hit her harder. She has written about it in several writing projects at school. She does know that the Dr. could not save him and he too was very sick. His ashes sit with Maggie on our mantle with a picture of them.
We have 3 dogs now and it never gets easier. We are animal lovers.
I did use a book from the library about Doggy Heaven which helped.
I am sorry for your loss. In neither instance did we say that the dogs were "put to sleep" or anything like that. We explained the life cycle and that death is a part of it.
TF