좀 늦었지만 답변 올립니다. ㅎ
http://codex.wordpress.org/Template_Tags/wp_list_categories 를 보니깐 wp_list_categories 함수에 다음같은 인자가 있군요.
depth
(integer) This parameter controls how many levels in the hierarchy of Categories are to be included in the list of Categories. The default value is 0 (display all Categories and their children). This parameter added at Version 2.5
* 0 – All Categories and child Categories (Default).
* -1 – All Categories displayed in flat (no indent) form (overrides hierarchical).
* 1 – Show only top level Categories
* n – Value of n (some number) specifies the depth (or level) to descend in displaying Categories
depth=1 이면 child category는 리스팅 되지 않네요.
wp_list_categories(‘depth=1’); 정도로 사용하시면 되겠습니다. ㅎ