This is a great subject and difficult for many families. Young children, even adolescents don’t always have a full grip on what a disease like Alzheimer’s entails. You can also start by saying “Grandpa / Grandma has an illness that makes it hard to remember things, but it doesn’t mean that I don’t love you if I forget something.”
It’s healthy to help them know that their anger or sadness about the situation is natural and okay. When you’re able, spending time with your grandchildren can help you both stay engaged mentally and emotionally.