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 after Winston has left the Parsons' apartment.