Whats worse? Your daughter finding out you do the same naughty little pocket shuffle that every man on earth does or that you and every other male in that kingdom hall is lying about not doing it....
Welcome to telling the truth about masturbation. Simple human anatomy and biology dictates that if you didnt 'choke the chicken' you would wake up to a genocide in your pants every morning.
Telling the truth in life is what is important and the important thing for your daughter to know is that living a lie is not really living!
It is a normal, healthy thing. Dont let anyone lie to you otherwise.