Count immediate child div elements using jQuery

I have the following HTML node structure:

<div id="foo">
  <div id="bar"></div>
  <div id="baz">
    <div id="biz"></div>
  </div>
  <span></span>
</div>

How do I count the number of immediate children of foo, that are of type div? In the example above, the result should be two (bar and baz).