Print periphery of a binary tree
I think that printChild() would be better named printDescendantLeaves().  Also, printChild() always works left-to-right, which spoils your clockwise traversal.

For flexibility, consider taking a visitor instead of hard-coding System.out.println() everywhere.  Alternatively, implement your traversal as an Iterator (which is admittedly tricky to do recursively since Java doesn't support yield).

"Periphery" is misspelled.