You are editing the class but they are also wrapped in divs that is getting affected by the parent theme as Miss Christine said.
Add this to your child theme. I tried it in firebug.
#site-navigation {
margin: 0px;
}
#masthead {padding-top: 10px} *(change the number of pixels so that you get any desired space.
As Andrew suggested, please take some time to understand CSS better. And if you learn firebug as I suggested, you will be able to see if those changes are taking place.