
Out of love and concern for your child, would you please read this short article?
http://www.ccef.org/its-all-about-me-problem-masturbation
It clearly outlines what the root problems are that lead to masturbation, gives hope through the gospel that victory can be won, and offers practical advice about how to deal with temptations.
Your son or daughter might hate--or loathe--the idea of you speaking with them about such a "private" matter...But if we remain silent about it as parents, we are not doing our part to raise them "in the discipline and instruction of the Lord" (Eph. 6). And we must remember that no sin is so "private" as to fall outside the authority of a parent to speak truth into their child's life.
Of course, this could take some easing in to. If you've never once even talked about lust with your child, it's going to be difficult to launch into this type of even more personal conversation. But start by voicing your own previous struggles with lust, by praying together for them, etc. And slowly work your way up to it.