So, I had the talk tonight with the 2 older girls. It went something like this:
Me: "Do you know why Mommy and Daddy and Grandma are shaving their heads?"
Daughters: (head shaking no)
Me: "Well, Grandma is really sick and she has to take some medicine to help her to feel a little better. This medicine will make her hair fall out, so we are going to shave our heads to support Grandma. "
(Now, this is where I prepare for the tears and the questions. Instead, I got:)
Daughter #1: "Oh, okay. Can I have a drink?"
Well. I'm glad they absorbed it all. And I'm glad that I have stressed about this conversation for the last month! :)