The average bra size that Pre Teens wear is between a 30 cup size and a 36 cup size in bras.
The cup size for a Pre Teen also ranges from A to B cup size for bras and Pre Teens will be wearing a trainer bra which usually comes in sizes in XS and XL sizes.
So you will be picking out a bra in sizes XS and XL size for a Pre Teen instead of through cup size.