it's my duty ...It's my duty to lok after the children.可以换成以下这样的表达吗It's my duty that I must look after the children.

it's my duty ...
It's my duty to lok after the children.
可以换成以下这样的表达吗
It's my duty that I must look after the children.

可以的

可以，后者就可以看做同位语从句

应该可以