Haha! Do not worry, I did this when I was her age and TOTALLY grew out of it. All little girls do it. They dont really know if its wrong. After about a year shell stop and forget it even happened she will probably start again at 12 or 13.
Actually it's perfectly normal at her age, and younger (I know I was "playing with myself" at that age)! So long as she's not doing it publicly or with adult company, you needn't worry - and if so, just tell her it's rude to do that in front of people (without getting carried away)
It is normal for children who have discovered themselves to continue. If you catch her in private places masturbating after telling her it is a private thing, (which it is) then either you will have to discontinue telling her it is a private thing or let her alone when you catch her. As far as her sneaking looks at the movies you watch, you will have to be more vigilant and more secure. Or, stop watching them. My daughter masturbates quite frequently, and I have caught her several times, before I started knocking before entering. Eve after I enter I know what she was doing. I just go about what I came in there for and leave.