How do I tell my children their grandad doesn't have long left?

My dad was diagnosed December last year with lung and bowel cancer. we was told he had up to 3years if his chemo and immunotherapy worked weeks if it didn't. Everything was going ok until 4weeks a go he was losing feeling in his legs and falling a lot so they stopped his chemo to help him gain some strength and weight back. His appointment on Monday wasn't good, his body can no longer take anymore chemo and we have been given weeks as a time frame.

I'm struggling myself with my mental health since my dad's diagnosis. I'm putting on a smile and brave face around everyone but inside I'm breaking. My sons are 11 and 9 years old. They knew grandad was poorly with cancer and I did say as long as his medicine works he will start to feel better but now they have noticed he does look more ill and started asking questions that I don't even no how to answer myself. I don't want to keep it from them, they have always been close to their grandad. He's the rock of the family. I'm the oldest and the only girl and my dad as always been the one I go to for everything. And every night I go to bed I'm waiting for that call that I don't want, but I know its coming :'(