1.John ,who is good at cooking,has been arranged to prepare the food for the guests by me.
who is good at cooking,这一部分是英语中的一种用法,叫做插入语,是对句子中的某一部分（多为名词,这里是John）做出的解释说明.在语法中做状语,在句意上仅仅是起到额外说明的作用,或者说在句意上是可有可无的,即使去掉,也并不影响主句的整体意思.
2.The children won't be allowed to enter the room by their mather until they wipe their shoes.
until they wipe their shoes是条件状语从句,不管主句怎样变换,只要把状语部分放在最后,就一定没错.
总结：这两句变被动语态的知识点主要也就是在“to do不定式”和“插入语”及“状语”对答题者的干扰.
其实只要先挑出状语后,先把它放在一边,等把主句都变完,再把状语放在最后,就可以了.
不定式：do to do变被动态也很简单,只要把前面的动词变被动,再原封不动地接上后面的to do就可以了,即：“do to do”变被动就是：“be done to do”
我的解释就这么多了,若有哪里看不懂就再问我吧o(∩_∩)o