The term "tween" stands for "in between" being a teenager and a child, and generally refers to prepubescent kids who are 9-12 years old. It refers to the breaking point of a child when he or she rejects more childlike images and associations and aspires to be more like a teen.
Here's an example of its usage: "You should add 'crdtchck' as shorthand for 'Credit Check' because it is an important topic that will become more relevant for GenY and tweens. Tweeners seem to be texting all the time, don't you think?" Yes I do ;-)
Click on "more info" below for a list of texting shorthand!