个人以为楼上这些都是中式英语,因为你这句话实质上的意思是说父母（的行为）对孩子的行为有很大的影响.所以主语用parents显然都是不合理的.
直接字面意思翻译个人觉得比较合理的是What parents do has a great impact on their children's behavior.
或者直接说Parents should take responsibility for children's behavior.即父母应当对孩子的行为负责.看语境选择吧.能否用have a great effect造句？Whatparents do has a great effect on their children's behavior.