D.N.
I would maybe get them up a little early so they can spend time with her before school. Let them know she will not be there when they get home. I would also tell them today about it. There will be questions. The 6 yr old may understand more but both may be confused. Tell them why she is going away. They can see she cannot walk. They can see she cannot do a lot of things. Don;t hide anything, let them say goodbye the night before and let them spend a little time with her in the morning. Tell them what is going to happen.
We had a cat that got very sick in 2011. My husband took her to the doc and we did try to save her. The doc wanted to give her a transfusion but I just had no way to pay for it. The day after she seemed a bit better and we took her outside to wash her up a bit. It was a really nice day so she also got some air. Unfortunately, she died while we were washing her. My then 3 yr old and one of my older girls was with me and saw her start to have trouble breathing and chokiing and then she got really pale. She was a black cat so it was especially noticable. Even though I explained everything and all 3 of my girls went with to the vet to take her for cremation, my 3 yr old asked for several weeks when she was coming home. It takes time to understand.