You can convince them by earning your own money and spending it on those items. My daughter (age 5) wears Justice, abercromie and Hurley, but I also get them second hand and at garage sales (I usually pay 50¢-$1 per article of clothing). Brands are important to her, but I refuse to pay full price for them.
Be flexible. Can you find then items you are looking for at a consignment shop such as Plato's Closet? Will your parents be willing to give you a clothing allowance and you decide where to spend it? For example, if they are willing to spend $100 per year on new clothing for you, would they be willing to just give you the $100 and if you decide to blow it on two things then so be it (bearing in mind, of course that you don't get any more money for clothing later)? Perhaps they would be willing to pay a certain amount ($15 for a pair of jeans, for example) if you were willing to make up the difference.