How to get these two divs side-by-side?

Solution 1:

Since div's by default are block elements - meaning they will occupy full available width, try using -

display:inline-block;

The div is now rendered inline i.e. does not disrupt flow of elements, but will still be treated as a block element.

I find this technique easier than wrestling with floats.

See this tutorial for more - http://learnlayout.com/inline-block.html. I would recommend even the previous articles that lead up to that one. (No, I did not write it)

Solution 2:

#parent_div_1, #parent_div_2, #parent_div_3 {
  width: 100px;
  height: 100px;
  border: 1px solid red;
  margin-right: 10px;
  float: left;
}
.child_div_1 {
  float: left;
  margin-right: 5px;
}

Check working example at http://jsfiddle.net/c6242/1/