(or preteen) Used for children just before their teens, "be tween " childhood and being teenagers, roughly 8-12 years of age.

a child who is approaching their teens but isn't there yet

a kid who is not yet a teenager, but too big to be a little kid anymore

Short for in-betweening. Actually, in Macromedia Flash for example, the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image.

one who is in between butch and femme, and can take on either role; a switch