Make child divs expand to fill parent div's width

Answer 1

You can add these three properties to the .child CSS rule:

width:33%;
float:left;    
box-sizing: border-box;

The last line makes sure it will also work when you add borders, padding and margin to the boxes.

ONLINE DEMO

Ps: not directly related but there is also an error in the border-bottom for parent, corrected in fiddle above. When you use non-0 value you need to specify unit:

    border-bottom:1px;