I have the same issue with Charlotte to a certain extent (She just turned 3)

Firstly, we found she was getting irritated, smelly etc. so I started adding tea tree to her baths - every night, 6-10 drops in the bath. This has solved that problem.
I also found this has helped with the odour problem on her hands too.

Secondly, I try and make sure she has knickers and pants on as much as possible, she isn't much of a stick her hands down her pants, but will go for the fanny when it is exposed (Opportunist? lol)

We have found we do need to be quite firm about it though - the stop playing with it, take your fingers away etc, I most often use the phrase "Stop playing with your fanny with dirty fingers, you will make it sick and itchy". She seems to be understanding this, whilst having me trying to preserve the 'this is your body, it isn't wrong' etc theory.

I am very very strict with her about the putting things in it. Which I struggled with (as she has seen me use tampons many times), so trying to explain to her that she shouldn't/we don't stick things in it, was difficult. But I have to be able to know she knows that nothing is to go in it at all.

So it has begun to get better, but she still plays with it more than I would like, and is roughly (pulls at it, tugging at it etc) playing with it too, but I keep reinforcing with things like "mummy doesn't do that to her fanny", "careful, or you will make it sore and itchy...".

HTH