Hello, I think it is lovely that you are so concerned about your Nanna. I am afraid I am ancient, in my 50's, so that's really old when you are 9.
I have a few ideas if that's OK. You are welcome here because we do understand as many of us have much loved children and grandchildren who we try to help, so please feel free to talk to us oldies.
Can you talk to your teacher? There will be a school nurse at your school, you don't have to be poorly to talk to them, you can talk to them about Nanny, there will be a Special needs co-ordinator at your school, and you don't need to have any special needs yourself, your special need could be because you have a very special nanna who is poorly.
It may be that it is a dinner lady who you really like and get on well with, you could ask for a chat with them. I would bet that the adults could find other people in your school and in other schools who are going through exactly the same thing you are.
You could be the school dementia champion, you are already probably the youngest person on this site, how cool would it be for you to be a part of putting other children in your area together to talk about dementia and how it hurts you. It does hurt, it hurts us oldies and we're supposed to be able to understand it better, for you young people, it would hurt more.
I know your parent is fabulous and supports you, I feel sure you will show your mum this thread so maybe she might go to the school with you to help you link up with other young people if she thinks it might be a good idea.
I think it is wonderful that you care so much and you sound like a fabulous caring young person. We oldies will be here for you too.