这个句子大家应该都能看懂，看看怎么分析。
If you promise not to believe everything your child says happens at school, I'll promise not to believe everything he says happens at home.
就分析第一个，
If you promise not to believe everything (which/that) your child says () happens at school
那个Everything是定语从句嘛？定语从句的成份可以省略的？那happens的从句是什么从句，也是定语从句？
------解决方案--------------------
那个Everything是定语从句嘛？
是定语从句，这里是your child says做宾语
但是也是做happens at school的主语。
但是我宁愿理解为 where happens at school
一般引导定语从句的关系代词有：who , whom , whose , which , that 等。
关系代词在定语从句中作宾语时可以省略，但也有几种特殊情况是可以省略的
具体参考http://edu.sina.com.cn/en/2004-06-28/23121.html