You need to create a pagedatacollection or you set the property "PageLink" for the pagetree control to a specific page in episerver for instance set the PageLink property to PageReference.StartPage and you will get the children from the startpage of you website. In CMS 6 the webcontrol binds data automaticly but if you are working with earlier versions of episerver you will need to call DataBind() for that control.

PageTreeControlID.DataBind();