For me, words like either "bottom" or "front bottom" are out. Boys have a name for theirs that isn't borrowed from some other part of the body, and girls should have the same. If you're going to say "front bottom", why not "downstairs lips"? Yuk. And "bottom" is even worse. Not only would it cause confusion in your own family, it would cause confusion for your dd at school where she's quite likely to misunderstand what other kids are talking about.
Poncy words like "vagina" and "vulva" and "genitalia" are also out. Well out! You don't want your kids talking like a medical dictionary.
So you're left with made-up words. Personally I don't like words that sound too much like what boys use. "Twinkle" sounds a bit too much like "winkle" which was probably the most popular word for a boy's thing before "willy" took over. In fact, I don't like anything with the "ingle" or "inkle" sound, because it reminds us of something that dangles. When I was at school boys used to say "At what angle does your dingle dangle?" Answer: "It depends on the heat of the meat".
Boys have "dingle" and "dinkle" and "winkle" and "tinkle". I've even heard "chinkle-chankle". I think girls should have something with a different sound. So you're left with a few suggested by posters here "foo foo", "fanjo" (which I love!), "mimi", "woo woo", "flower", "pee-pee" (yuk), "fallulah", "noony", "minkie", "lulu".
The trouble is that mine was called so many different things by my mum that I can't remember any of them. To begin with it was baby names, just noises basically, like "uh-uh", "buh-buh", "ee-ee". Later she used lady names "mabel", "petunia". "lady clementine", and a host of others. And I'm afraid I continued the tradition with dd.
Me, if I had to pick something permanent, I'd go for either "fanjo" or "fallulah" (I was about to say it's a toss up between them, but that's probably inappropriate). Also "zezette" sounds quite classy. The only thing you shouldn't call it is what dh calls mine: "your vertical grin".