The heads seem too tall... You should try to make them around the same length as the upper arm. Look in a mirror, hold your arm up next to your head, and bend your elbow to see that the head shouldn't be longer. You could also make the heads a bit wider to even things out... It's not like it's really wrong to give characters bigger heads than realistically expected. I'd also move the ears closer to the center, though it's not necessarily wrong to have them where they are. The girl's ear seems too high, though. Here's a slightly old tutorial I made explaining head/face stuff:
http://cloudflash.net/arttips.php (click the "Next" button on the Flash thing).
The girl's legs seem a bit long. If she was sitting with her knees up to her chest, the knees would reach around where her collar bone is. She also seems to be missing her butt. Or it's like... not connected to her back at all.
The guy doesn't quite look like he's sitting... For the leg to our right, the lower leg seems a bit too long maybe. I'm no expert on poses, so I dunno what else to say about the legs.
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