G.B.
Put her in ladies underwear.
Girls size 8-10 is just about the same size around as a ladies X-Small, a gilrs 12-14 is almost identical in size to a ladies size small and sometimes even a medium, a girls pre-teen or size 16 is larger than a ladies size small and most mediums. They are sometimes similar in size to a ladies large.
I work around girls cloths and ladies shorts and pants on a daily basis and I can hold up a pair of shorts identical in cut and style that are a girls 12-14 on top of a ladies size small and the only difference is the legs of the shorts are about 1/2 inch wider and the crotch seam in about 1/4" to 1/2" longer. Not much difference at all unless you are a little girl who is going through puberty and getting her adult hips and rounded shape. Then the adult shorts just suddenly fit more comfortable.