I figured it out!
The parent should be overflow:auto;
The .child should be position:relative;
The .child-menu should be position:fixed; with NO top or left positioning.
If you do this, it will keep it it inline with the content.
If you need to move the child-menu use margins and not top or left. Example margin-left:-100px;
EDIT
As it seems people still use this, please note that you will have to use javascript to move the fixed items as the page scrolls.