Where in 1984 does it refer to children turning in their parents to big brother for thought-crimes?

We are
definitely told in the book that children are encouraged to turn their parents in if the parents
are having "wrong" thoughts.  In fact, we are told that the newspaper carried stories
about kids who did this just about every week.  This is supposed to refer to how, in communist
countries, kids were encouraged to do this very thing in real life.

You can
find evidence for this in the text towards the end of Chapter 2.  Since I probably do not have
the same edition of the book that you do, there is no real use in my giving a page number, but
it is just afterhas left the Parsons' apartment.