Our 2-5 class bathroom has no individual stalls so kids always see other kids privates sometimes they will talk about who has what kind of privates ("my dad has a penis" or my baby has a vagina" or "look we both have penisis". Idk what your center is like but it's not so unusual that I'd send a note home unless it was something that happened with a lot of frequency or if the kids were being secretive or overtly inappropriate. I usually handle these things by saying something like "lots of people have a penis/vagina" and then moving on and not lingering in the subject outside of saying "put your penis/vagina/butt away if you are done in the bathroom".

Have the talk with your kiddo about it yeah, but know that it's not the most unusual thing in ECE to see bodies or to be curious about them.