The problem is that a space is rendered between your child divs. The issue is neither margin nor padding - it's actually caused by the whitespace between your div tags in the HTML code!
Place the tags directly adjacent to eachother without any whitespace and your problem will be solved ;)
<!--whitespace in HTML = renders as a space between the divs-->
<div></div>
<div></div>
<!--no whitespace in HTML = renders edge to edge-->
<div></div><div><div>