? About how to explain being transgender to a 6 year old

So my hubby & I just found out today that my stepson's other stepmother (hubs & ex got divorced when his ex came out as gay) is transgender & will be starting meds to transition her body from female to male, hubs found out on Facebook so there haven't been any discussions w/ his ex about if my stepson has been told yet, if they're changing pronouns yet, if she's going to go by another name etc. hubs & the ex's relationship is fairly friendly & respectful but they're not buddies, we don't hang out w/ them or anything so hubs is kind of freaking out about how this is all going to play out, how to talk to his ex about it & of course how this will affect his kiddo, I wanted to ask if there's anyone who's transgender on here or anyone who's close to someone who's transgender if u have any advice for how we should deal w/ this w/ my stepson, I wanna make sure we're respectful in regards to how they might be explaining things in the other household while addressing any confusion my stepson might have, please only positive helpful responses, I don't need to hear from anyone who is anti LGBTQ! Thanks in advance everyone