Oh honey, I'm so, so sorry I had my fingers crossed all day for you and your poor kitty!! I know you must be devastated (I would be, too) but you know what the right decision is, and you can take some small comfort in knowing that she's had a very comfortable life and has been loved by you very, very much. I know that doesn't make it easier, but FWIW I think she knows and appreciates the life she's had with you.
Definitely explain to and include DD, I'll never forgive my dad for having our cat put to sleep and burying him while I was at school... I got home and he's standing in the yard with a shovel and wondered why I lost my mind and nearly brained him with it :/ It will be hard, but I think in the long run it'll be easier on DD than if her beloved kitty were to just disappear, kwim?

I'm so, so sorry for you, Liviam. Please accept my sympathy and know that my heart is hurting along with yours