第一句不可以加who,这句话是定语从句,magazine是物,who是对人提问的,这句话其实是magazine后省略了which 或者that.加上这两个,这句话也是对的.
至于你问的为什么不用what,因为这句话是定语从句,定语从句的连接词只能是which ,that ,who ,where这些,我看到what 引导的只有主语从句这些,例如:What i really wanted is that beautiful skirt.
第一句改成主语从句的话应该是:What i sent to the wrong address was the magazine I ordered.
第二句Children是人,所以就用who