masturbation-the only word for it!!

[Empress]

Our little girl (4) keeps wriggling around on the sofa when she's watching a film, we've tried telling her its a private thing to do , but she loves doing it, & finds it hard to stop! We don't want to make a big deal out of it for obvious reasons, but have told her that its not something she should do e.g at other peoples houses.
any ideas on how to get it under control without making her think its dirty or shameful?