Tweencore is an aesthetic that's a combination of Kidcore and Teencore, Maintaining the nostalgic and "childish" themes but also embodying teen spirit. It tends to borrow heavily from Animecore (especially girlier animes like Sailor Moon and Fruits Basket), Scenecore, and Webcore. Its aesthetic is centered around the preadolescent period of the ages of 9 to 12, so while there is certainly a lot of pink involved (like you'd expect from Kidcore for girls), it's a slightly refined usage of the color pink. There's also an emphasis on Disney Show sitcoms like Hannah Montana and Disney princesses as well.
Tweencore aesthetics involve lots of pinks and purples, pretty anime girls, old internet messengers from a bygone era (especially MSN Messanger and AIM), Disney princesses, and a developing rebellious spirit. One may also see Boy Band/Pop Princess/teen idol images pop up in the Tweencore aesthetic as well.
Tweencore fashion involves lots of miniskirts, tank tops, babydoll-style shirts, sneakers, girly socks, and involves pinks and purples, flowers, rainbows, and an overall "girly" sense of fashion. There may even be the odd occasion of somebody in Tweencore having braces (which really helps to sell the illusion one would go for with Tweencore).