I just read an opinion on this subject and felt I needed to share my recent knowledge with you.  Girls can go for a LONG time and still be able to wear "little girls" sz. clothing but this might not be your best fit.  My daughter is approx. a sz. 12-14 girls and lately we have discovered after spending  hours in the girls section that we needed to be in juniors even though the girls dept. still had clothing sizes too big for her.  The same thing happened with my son.  Here is my humble breakdown on sizes for kids that are reaching preteen/teen...

Girls sz.10-12=Jr. sz.0-1
        sz.12-14=Jr. sz.1-2

Boys 14-16 or xl=adult md.
adult smalls are definately smaller than a boys XL 14-16
good luck!