I had to tell my 5-6 kids in Kindergarten last year that I had cancer and would be gone the last two months to do treatment. When I came back with a hat this fall, all kids knew. Here's how I told them. I said I had a back sickness under my arm and on my chest wall. That I was going to take a really strong medicine to make it all go away. That medicine was so strong that my hair would fall out and I wouldn't feel so good. But that soon I would feel better and the hair would grow back. That's all I needed to say and all kids were great.
My oldest is an RN. I had to tell her on the phone. Tears and then good. She drove the doctors crazy with questions, but it was great to have another brain and ear. I told my college girls the same thing - more detailed. My dear hubby was with me all the way, at diagnosis, appts, and never waivered. Few tears when I had tears but not much to say. Asked a few Q when he could see I was overwhelmed.
Something we are taught as guidance counselors, is to never try to hide info from kids. They definitely know that something is going on and feel left out and confused if you aren't upfront. The older they are the more devalued they will feel even though we think we are loving them more by keeping something from them. Tell them the facts. You only have to add the details to the level they can understand.