Hmm. I like the style, it's quite cute.
For your boy, I drew what was wrong:
The white lines depict a rough outline of where you should place highlights in his hair. Heads are round, and shading has to reflect that, and how light passes around it. It's usually done with a "halo" of highlights.
The yellow circle shows where a stray line on the eye was bugging me. It'd look better more symmetrical to the other eye.
The red lines indicate that your anatomy is very off. The clavicles are too high and far apart (in a young boy, they wouldn't be
that visible anyway). The neck is too short. One shoulder ends abruptly under the shirt. The shoulder muscles are too vertically big. The collar of the neck does not hang appropriately around him.
As for the girl, the face is fine. The lips look strange though. The rest of the body's proportions are ... difficult to look at for me. There's a lot wrong with it. I'd draw it out but I have to be somewhere in a few minutes. My recommendation is to try to look at some references for ballerinas and such. I understand that your characters are "thicker" and more cartoony, but the proportions look more novice than cartoonish.
They're not bad; they're actually quite cute. You just need to work on your body blocking.