I'm with the general concensus here here, I think the kids need to be carefully told also.
Back when I had left home, to go in the US Air Force, my Dad was gone to Army Reserves Summer Camp and my (so called) brother was away with a cousin, my Mom had our dog "Missy" taken to the vet to be put down.
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Now, the thing is, there wasn't anything wrong with her. my Mom just thought that she was a bit much to deal with because she had started climbing the fence and getting out of the yard.
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Yeah, it was a pain in the butt, to have to run her down, praying that she didn't get run over or something. Wen we all got back home, my Mom said that Missy had apparently climbed the fence and took off. Well, for months, every time I went out, I would keep my eye out for her. Then, one day, I thought I saw her on the other side of Little Rock and tried to get her to come to me. Only thing is, it wasn't her! When I got home, I told my parents what had happened and my Mom decided to tell us what she'd done. Now, while I was MAD, my Dad went out in the back yard and stayed out nthere for a couple of hours. When he came back in, my Mom knew she'd messed up big time and she promised NEVER to do anything like that again! And, to my knowledge, she didn't. So, YES, please try to tell the kids about the cat's condition. I know it will be hard, but, it will be better in the long run.