Having to Euthanize Dog, How to Tell 2 Yr. Old

Unfortunatly we are about to have to put our dog, who was my wedding gift from my husband, down. He has a tumor on the side of his neck that is malignant. Anyway, the thing is, he acts fine most of the time which will make it hard for my daughter to understand that he is sick. She knows that Dusty has a boo boo and that he has had to go to the dr. but that is about it. I am really dreading this, but I know I need to tell her the truth. I just don't know how far I should really get into it. Should I tell her what is going to happen and let her say goodbye to him or do I just take him to the vet while she isn't home and explain it after? My daughter and Dusty are very close and I know she is going to miss him terribly. Once when I took him to get groomed my daughter was with me and she freaked when we left him. I had to drag her out of the place and she cried and called for him and looked for him all afternoon until we went and picked him up. I was hoping I wouldn't have to deal with this for awhile yet. Any advice would be great!