Hi Azirial,
If it were me, and the kids are fairly young, I'd probably just give a very generic 'hormone issues' explanation and leave the exact details till they're older. Plenty of cis women have naturally small breasts, and some have varying levels of facial hair due to medical issues (PCOS being the first one that comes to mind). I wouldn't claim a specific issue that I didn't have, just tell them the minimum to satisfy their curiosity if they ask about it.
Regardless of what you decide though, this is something you should discuss with your boyfriend and try to reach an agreement on how to approach it before you discuss with the kids. Maybe he has a strong preference on what to do, or would want to be the one to tell them? You won't know unless you discuss it.
-Megan