-Lay down the rules you want them to follow and make sure you stick to them. If on day one you have asked your kid not to wear a sleeveless top, make sure you do not say a 'yes' the second day. This will firstly confuse the child and secondly, he/she will start taking things lightly and for granted. Be strict with the rules and make sure you yourself stick to them!
-Make your child aware that is important to wear clothes that are thick and long. Boys need to wear clothes that reach at least up to their knees or till their ankles and girls need to keep their entire body covered except for their face, hands and feet. When this is told to the child right from the beginning, he/she will never choose an apparel that will not fulfill the above mentioned conditions.
Read More : http://www.storeboard.com/blogs/fashion/the-art-of-choosing-appropriate-clothes-how-to-instill-modesty-in-islamic-children/346454