My LO will put her hands down there during diaper changes too. But she doesn't try to get through her pants or her diaper.
Maybe if you put her in a onesie for a little bit she wont be able to get through it? But I don't see the problem.
We were at family's house for Christmas when she was doing it. It's not a problem "per se" but I guess my question is for when/if it happens in a public situation again.
With my nephew my sister always told him 'that's private' then redirected him. If he kept on, she would tell him like you did with a firm 'no, that is private.'
If they don't know what private means you add that in. 'That's private, we don't do that in front of people' or something like that.
Eventually he got it and was all about privacy. He loved to go into the bathroom and just stand there yelling i need privacy!...literally just stood there because he didnt use the bathroom yet..he was still in diapers..Haha!
Your LO will get it eventually. GL.