I would make sure your teen is on the same page and is sitting there when you tell them. I would keep very calm and positive.
I would say, "Daddy is very sick. We need to move away for a little while until he feels better." That's it. I am sure they will have tons of questions. Answer them in short and simple, but honest.
I work with kids for a living. When parents tell them too much information they get confused and stressed.
I would try to make sure you stick to a healthy schedule. The need consistency. Same same same.
Make it fun. Can they pick colors for their new room? Check out new ice cream shops in the town? New parks? something... and hopefully the teen will be super supportive and helpful!!!!
Maybe have a picture of dad in their bedroom. And tell them good stories about him and who he was. So, they know they came from good. When ever opportunity presents itself let them know how important good choices are.
Always be honest, but at that young age simplify.
Btw, I moved very fast. My son was not yet 3. It was the best thing I ever did!!! I am proud of myself for working/moving very smartly under such pain and stress. I was strong during the day, and cried myself to sleep. 1 year later I was in a good place. 2 years later I was in a really good place. My son is and was safe! thank god!
It is sooooo hard!!!!!!!!!!!!!!!!!!!!!!! Bless you!