It was not until midnight that the children were found not in their bedroom.
这句话是不是可以改写成
The children were found not in their bedroomnot until midnight.
这句是强调句吗
not until 是一个词组吧 ?

但是这样理解好像又通
It was not-- 这事一直是not
until midnight -- 直到午夜(变成yes/true)
that ,, 从句 具体说这件事