subpage menu

When on a parent page, I’d like a page navigation menu like so:

==Page 1
==Page 2
==Page 3

(i’d label Parents 1, 2, 3, but only page 3 has children.)

When I go to a parent page with children (i.e. if I open Page 3):

==Page 3
====Child 1
====Child 2
====Child 3

then when I go to a child under that (i.e. if I open Child 1)

==Child 1
====Grandchild 1
====Grandchild 2
====Grandchild 3

any code for this?