Good luck in the contest. I once won with a hand-drawn entry, and I have a cool medal as proof.
Here are the edits I suggest:
1.The hair is... awkward. Make sure they have a part, and that the hair is not too "flat".
2.Don't make the clothing too "Even Steven". If you notice, when somebody has pants or shirts on, it is not perfectly alligned.
3. the boy's shoes are pretty good, just don't make the heels too round.
On the girl, apparently you gave her flats. make them a bit smaller, not so huge.
4. I added sleeves on the girl's dress, since I am the type of person to wear sleeves, but you don't have to have them.