I'll admit, I could be totally wrong about this, but I would think it's best with older children to be less clinical and more personal. What I mean is, instead of focusing on terms like Gender Dysphoria, Transition, Transgender, etc. I would think it would be better to focus on what your life experience has been like all these years. For adult kids I think discovering that their parent has been secretly suffering all of their lives and hiding it to shield them from it builds some instant empathy. Once you've got it out there that this is what you're doing and why, then I think you can get into the more clinical discussions. Just my thoughts.
Oh and yeah I would definitely do it with them all together if you can to avoid that sibling rumor mill.