to expect
这里的结构是：疑问词+ 不定式,相当于一个从句,在这一句中是一个what引导的宾语从句,做know的宾语.可以写成 I've worked with children before,so I know what I except in my new job.
举一个更简单更容易懂的例子吧.
I don't know what to do.
相当于I don't know what I can do.
换成副词也可以,比如：I don't know where to go.
相当于I don't know where I can go.