How to acccess childs?

programming flash
1

Hello,

I have made a few movieclips on the stage with childs (non AS 3.0)
and now I would like to access these childs with the following script.

this.mcWebsiteNavigation.mcNavigationContact.addEventListener(MouseEvent.MOUSE_UP, showContact);

function showContact(eHandler:MouseEvent) {
    var req:URLRequest = new URLRequest("http://blog.apsadvertising.nl/#movie");
    navigateToURL(req, '_parent');         
}

and now I get the following error

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mcWebsiteClass/::frame60()

What am I doing wrong? I just made my move into AS 3.0 so I have got no clue.