Arte, I don't think there's too much you need to plan re the information for the kids. I think kids are usually happy to accept a simple but straightforward explanation like 'Uncle is very sick in a part of his brain, it makes him do and say strange things sometimes.' Let them know it's ok to feel confused or strange or sad (or whatever) if/when they witness those behaviours. It's possibly going to be easier to explain now that you know a specific illness is causing it than it was before when it was just bizarre/difficult/hurtful. As the disease progresses, what you need/choose to tell them will depend on how the situation is unfolding, I don't think you can plan that far ahead.

As much as you feel like you want to be able to make a plan, take some kind of action, change the outcome somehow, terminal illness is one of those things that are outside of the family's control. Beyond any practical help you can provide (meals, cleaning, errands, going with them to appts, offering to ring to update other family members so they don't have to tell the same news over and over, helping get necessary paperwork done) this often means just turning up, asking them how they are doing, letting them know you understand and care. But they will probably also want and need some "normal" in amongst everything else, like normal family stuff. It might not seem like much, but that's the glue that holds people together in hard times.