Container/Wrapper Div does not contain all content?

With my crystal ball, I will predict that your child divs are floated and your container is not. In this case, the container will not expand to fit its contents. Try floating your container and see what happens.

The crystal must have been dusty... However, the code you posted is not valid - you have content inside the head tag and a div outside the html tag. Is this how your page really looks, or is it just an error pasting the code into your question? Try cleaning up the code structure and see if it helps.

EDIT: Found the problem - it is a typo. You have <div="header"> - it should be <div id="header"> (note the missing 'id')

Try giving the clear:both to the parent div or put a div at the end of it:

<style type="text/css">
 .clear{clear:both;}
</style>

Possibility One:

<div id="parent">
  <div id="child1">Some Content</div>
  <div id="child2">Some Content</div>
  <div id="child3">Some Content</div>

  <div class="clear"></div>
</div>

Possibility Two:

<div id="parent" class="clear">
  <div id="child1">Some Content</div>
  <div id="child2">Some Content</div>
  <div id="child3">Some Content</div>
</div>

A great tool: http://validator.w3.org/ - this will tell you if there is markup errors and on what lines.