Hi, B.,
Now is the perfect time to tell your daughter (during a bath, perhaps) that any part of her body that is covered by a bathing suit belongs to her! They are private, and no one except mom, dad, and the doctor (when mom is with her) are allowed to touch her there. Let her know that if anyone ever touches her private parts, she should tell them NO or STOP in her big girl voice, and she should tell mommy right away. This goes for other children telling her to pull her pants down, too. Yes, children at this age are curious, but they still need to be taught appropriate behavior. No matter who it is, no matter what they say, she should always tell mommy about it. (You should keep in mind that molesters often tell the child that if she tells anyone they'll hurt her mommy. Make sure she understands she should always tell you. Mommy will never be mad at her about it.) Be sure to speak in a calm, matter of fact voice so as not to freak her out. And don't do it just once, you'll need to have this talk every so often as a reminder. Also let her know that if she ever feels uncomfortable when someone hugs or touches her, that she has the right to ask them to stop. It's her body, she owns it.
And, YES please DO use the correct terminology when teaching her what her private parts are called!! I cannot believe how many women call their vulva a vagina! "Well, I was looking at my vagina the other day..." Sweetie, unless you're in certain yoga positions, you are NOT looking at your vagina! Good grief!
Sorry for the lengthy response - hope it helps a little. J.