I would highly recommend getting them the book, The Catholic Boy's Guide or the Catholic Girl's Guide by Father Lasance. This book helped me a lot not only in regarding purity, but in many other ways too...
When children grow to be teenagers they want to know the reasons for everything. Don't be afraid to explain to them why they ought to dress modestly so that they can avoid near occasions of sin. My mother had us read the book, Dressing with Dignity, when I was a teenager, and that also helped.
Oh, and one more thing... Good example is one of the best ways to teach children purity. They will tend to look up to their parents and friends as examples in this. So, be careful of their friendships, and make to have a support group for them who encourages them to dress modestly. Kids hate to always be told what not to do. So, when they dress or ac right, don't forget to tell them that you are grateful that they do so. This will help cement a good connection to purity for them.