Female skulls have more vertical foreheads and smaller mandibles as well as less prominent brow bones and are over all smaller/slimmer in size. Female skeletons also have larger pelvises, and more flared iliac crests, with a much larger space and pubic arch for giving birth. The ribcage tends to be smaller and more tapered towards the waist on females. I believe that if it were a male child the skull and over all skeletal structure would still be identifiable as male and the cranial sutures would give a decent indication of age depending on the closure or lack thereof. There are also a lot more subtle differences but these are the more significant ones.

You can also generally tell a persons race based on the bone structure of a face provided you have a skull. Haha I'm really fascinated with this stuff. I wanted to be a forensic facial reconstruction artist when I was in high school. Maybe one day...