All of my kids touch. My boys did it as soon as they figured out it was there. That's the first place my 15 month olds hands go when the diaper comes off. It's totally normal.
What I tell my boys (they tend to more then my daughter) is, while I know that feels good -- it's private. If you want to continue you need to do so in the privacy of your own room. I've never actually had my child leave the room - usually they don't realize their hand has wandered there. So, I think I would say the same to your daughter. Basically telling her it's not wrong, not dirty but it is private.
If you're concerned you can also stress to her that no one else should be touching her there - unless it's Mommy/Daddy helping her wipe or clean up.. or a doctor with Mommy/Daddy present. Say it pleasantly - just informational. It will sink in as you repeat it.
Good luck Momma.