Sister act: Ashley and Jessica Simpson are teaming up to launch a fashion line ...

With an endless supply of fashion websites and style blogs at their fingertips, kids, it seems, are also developing their sartorial instincts earlier than ever before.

Now, as proof of the tween spending power, established labels such as Tommy Hilfiger and celebrities like Jessica Simpson and Justin Bieber are tapping into the market with diffusion lines and beauty products geared toward the under-18 set. 

Simpson, who already has a fashion empire worth nearly $1billion, has teamed up with younger sister Ashlee to launch a new line for girls between the ages of 7 and 16 that will include sportswear, denim and activewear.

The line, which will fall under the Jessica Simpson Collection umbrella, will hit stores in the fall.

Simpson told WWD: 'Creating a tween apparel collection was a natural extension for the brand.

'I’m excited to bring my sister, Ashlee, along as co-creative director of this division, because she brings a savvy rocker edge that defines today’s tweens.'

Tommy Hilfiger is also jumping aboard the pre-teen train. The American fashion giant recently announced that his Tommy Girl perfume will lend its name to a clothing line aimed at girls 12 to 18, to launch on July 15 at 150 Macy's locations throughout the U.S.

The designer's collection will be housed in its own mini-shop within the junior department of the store and will reportedly reflect Hilfiger’s signature preppy-with-a-twist aesthetic: Oxford shirts, polos, peacoats and denim.

'Younger audiences have shown a strong demand for the brand,' Gary Sheinbaum, chief executive officer of Tommy Hilfiger North America, told WWD.

'In leveraging our relationship with Macy’s to distribute Tommy Girl, we are able to offer product to this younger consumer and in more locations than we would through our retail business model.'

Lucrative market: Justin Bieber (left) has tapped into his teeny-bopper fan base with creating a bevy of beauty products, while Tommy Hilfiger (right) is set to launch his Tommy Girl line next month

'Let's be real, the way a girl smells is very important to a guy! I have such a deep connection with my fans, so creating a fragrance that I personally love is another way I can bring them closer to my world,' Bieber told WWD.

And he is not the only youngster actively marketing fashion to his peers. Mary-Kate and Ashley Olsen, one of the most famous examples, licensed their names to furniture and clothing lines for pre-teens in 1993.

Ashlee & Jessica Simpson Co-Design Tween Clothing Line

On Wednesday, Ash and Jess announced their plans to create a collection of tween apparel that will launch under Jessica Simpson’s wildly-successful lifestyle brand this fall. The line will come chockful of separates — such as denim, knits, hoodies, pullovers, graphic tees, outerwear, and footwear — carefully crafted by The Sisters Simpson.

“Creating a tween apparel collection was a natural extension for the brand.