You know Astrid, I'm a bit the same, I wouldn't want to call the whole area the vagina either. I didn't really have any awareness there was an 'inside' part to my bum til I was much older, it just wasn't relevant to me when I was a kid, iykwim. I think the whole area was just 'bottom' til I got older, but I'm sure if I had come home with a tale about someone else and my bottom, that would be enough for mum, til I was old enough to learn the specific names.
So far here, Riv has a willy, but I figure most adults know what a willy is anyway. It's the obscure girls bits names (like twinkle, wierd) that could be a problem I reckon.

And having a chuckle at your stories girls, kids can be so cute!
I might ask dh tonight what he and his brothers got up to as kids :P