We have just been through this with my DH. Our youngest admittedly was 4yrs old. We just went with as much information as they needed to know.
Daddy was going into hospital to fix his headaches. He will be well looked after and you can draw him a picture to make him feel better. It would be great if you help Nanna and Pop and Mum while he is getting better.
You will see him when he gets home.

Hope you are Ok MM, it's tough going