I would say you're just about perfect in most areas, one notable exception being your profiles (heads turned to the side). The chins aren't long enough; the faces look squished into the neck. Not an uncommon mistake.
This picture was good, though:

But only for the girl on the left. Your 3/4 views look a little iffy as well; I'd suggest doing studies of head turns in order to cover all the bases. It'll probably help your already-fantastic necks to look even better.
Also, I respectfully disagree with @ZombieOwl: when I put my hands on my hips in a mirror, my elbows look exactly like that. ^^; In addition, I think it may be one of those things where trying to be as realistic as possible actually does more harm than good.
Unless you are training to be a photo-realist, your job as a 2D artist is to convey 3D shapes in a 2D format as clearly as you can, and sometimes that means bending the rules. For example, a real fist looks mostly like a stub from the back, but many artists like to pull out the thumb and even the forefinger to help show what's happening. The same goes for a finger pointed directly at the viewer: sometimes pulling the fingertip up to show its surface is more easily-understood and dynamic than trying to copy the foreshortened mess that you actually get in real life.
In conclusion, a good cartoonist knows when to start studying, but a better cartoonist also knows when to stop.