My son is almost 7. All that I've told him is that we're a little different. It's harder for us to make friends. He's not very interested right now, so that's about as far as I've gone with it. I think you should keep it at their age level and like anything else, just be open and honest if they ask questions.
If she's going to have some therapies then he might ask more questions. It's not necessary to name it right now.
I wish your family well.