k first off your heads look a little squished. The girl should have a smaller neck.. If her shoulders are to be small like that then her neck should be way smaller..
prime example of this type is the art of lucky star.

If you want her to have a thicker neck then you will have to go to a fuller sized body like in kiddy grade

The neck is still kind of small..but not like in the lucky star pic.. same goes for small boys or smaller skinny men. Usually tho typically if made in the same photo you want to still make a neck of a boy bigger than a girls.. this also depends on types also though.
if full grown male of average or bigger build then you want a thicker neck than that of a boys.
Also eyes are only about one eyes width apart usually.
you should also take a few tutorials on basic anatomy.
these are merely suggestions however... art is only art if one loves the art they are doing.