Explaining it's not their fault
It's important to explain that they are not in any way responsible for grandpa’s condition. They are not the ones to blame for his sudden mood swings, and nor is grandpa responsible for his actions as a result of his dementia. Instead, his behavior is altered as a result of his condition. A good way to express this point is by finding age-appropriate books that talk about this topic, as the child might have an easier time understanding when told more through pictures and less with words. Similarly, there are tons of online resources that could help with this task, in case books are not easily obtainable.