Well, Elsiane covered a lot of things, but her redline has been deleted o___o!!!
I'll try to cover trying to draw it more childlike. I have a kawaii kawaii anime desu ne moe moe revolution yo~ style, so please keep that in mind if you don't agree with my points. These are just my opinions.

I actually thought it was a really short teenager (or a mother) when I first saw it rather than a child.
The things that made the child look too adult are:
The well defined Chin. Children usually have really round and pudgy faces. While you did give her some pudgy cheeks, her chin is really prominent and well defined so it makes her face look older.
Small eyes. I'm bad at drawing realism, but the eyes look really small in comparison to the head. Bigger eyes will make her seem more innocent. If she's squinting her eyes, maybe try to leave conveying of the emotions to the eyebrows too?
The nose is ok, but children usually don't have as well defined of a bridge of a nose.
http://upload.wikimedia.org/wikipedia/commons/8/82/Worried_little_girl.jpgFor me, when I draw the bridge of the nose, it usually makes them seem older.
The proportions would be OK if it were a teenager, but they seem a bit too long/big for a child. I know mine is not the best example though, sorry ><;;; But what usually helps is narrowing the shoulders a little
I really liked the perspective of the feet, they were really grounded! But the legs standing together like that makes the pose look kind of stiff, imo.
And also, I drew that ear in too high up and just now noticed it so please ignore it in my blueline >_____o
Overall, I really like how it kind of tells a story. you can tell the character is troubled. The proportions are great, but a bit too adult for a child ^^;; (Again, I'm not an expert) but I'm sorry for such a late blueline.