Two options:
1) Create a list element on the page, reference the parent page's navigation list (but you'd need to do this for each new page as I don't think there is a way to auto-reference a parent page element upon page creation)
2) Use a foreach loop and loop through the parent page's list element. The down side to this is that the order does not match the nav-order or the page order (I think it is sorted by guid, if I recall correctly). You could always use that to get the initial list, and then use javascript to re-sort it.