I am so sorry for your losses. Today has been a terrible day for you. I have you and your family in my thoughts and prayers. I hope your son feels better, it didn't help with the lack of compassion from his classmates. That's a terrible thing. I understand you wanting to keep the kitty's news to yourselves right now. Jupiter and your kitty were deeply loved and cared about. It was not your fault in any way. I'm sure he was glad to have you comfort him as he passed to the rainbow bridge.
I know it hurts and a life is a life, no matter how small. No life should be worth more or less than the next. I admire your and your family's compassion with this, especially your son at a young age. I hope you all feel better and start to have much better days coming up.