What I\'d like to do: create an MXML component with some children, then extend it via MXML to create a new component with more children, without losing the original set. In
In Flex 4, you will have to override your "mxmlContent" property setter in order to preserve your already defined children in a parent class
One of possible implementations of such a override is presented in the comment for this blog entry
Quick tip (Flex 4): Goodbye templates – hello mxmlContent
http://www.websector.de/blog/2009/10/02/quick-tip-flex-4-goodbye-templates-hello-mxmlcontent/