there is a property called numChildren on the UIComponent.
You can iterate using a loop and get each child using
public function getChildAt(index:int ):DisplayObject
Flex SDK Team
In Spark, because DOM elements are not all displayObjects, you should use getElementAt()/numElements/etc. The underlying player getChildAt() functions are still there, but they do not reflect the 'elements' of the spark container.
On raw UIComponents, the Child() functions are still the ones to use.