Anne Brashares the author of Sisterhood of the Traveling Pants doesn't write slang or brand names into her novels which gives the books a timeless quality.
Make sure, when writing, that you don't date your book by adding terminology that will be out of style in a few years. This could possibly be a turn off for young readers.
Read the classic stories. They don't talk about a brand name or use language that dates the book. You may not think it can be done, yet authors are doing it every day. Those are the books that will remain on library shelves for years to come.
Kids like to be able to relate to the main character in a story. If the character sounds too much like their parents talking they won't read the book. They also don't want to read about what was hip or fashionable ten years ago.
When you consider that it can take sometimes five years to get a book published, you realize that trends are not a good idea to write into your piece. What's hot today won't be five or especially ten years from now.
Describing a character's clothing in great detail isn't important. Let your reader imagine their own idea of what your characters are wearing.
Read lots of kids books, especially the great ones, the ones that have won awards and have had movies made from them. Examine the bones and disect the style in which the author wrote the book.
We don't want to imitate the author's work, but learn from their technique until we develop a style that makes the reader want to stay up all night to find out what happens next and then to mourn the loss of the end of a good book. That's what we strive to do as writers.
Happy writing. :o)