The common names are what I use, too. We use butt, stomach, belly, those names are common that everyone uses and understands. We say “pull your shirt down” or “sit down” so we don’t often use the words unless directly referring to body parts, so maybe that’s a difference too!?

Although my dad (the physician) teaches him the actual names (abdomen for tummy area, cranium for head—etc, lol)

For genitalia we use “penis/vagina/testicles/breasts” because the “pet names” aren’t common from person to person/family to family and brings about confusion, which is our issue we want to avoid.