At that age there is nothing sexual about it- however like play and learn said it feels good to them and they are exploring their own body.
I can understand why you wouldn't want her hand down her diaper for reasons of her getting them dirty from either urine or a bowel movement and I would discuss with the parents a way that you can handle it. Explaining that although it's normal for them to explore their bodies, with a diaper on and possibly putting her hand in pee or poo- it has to be addressed.
Good luck! I think the only thing you can do is be consistant in what ever you decide to do.