I have seen some things that suggest underground breeding stations that have existed for decades (or much longer). I don't know if that is what has really happened, but it is certainly possible. If true, there could be any number of children living in conditions that are indistinguishable from Hellraiser the movie.

Who says they're all missing from Australia?

no - those kids aren't being snatched off the streets in AUS cities. They are brought in from other nations with no records of their birth, especially following disasters where there is chaos and parents cannot be found.

They also come from grindingly poor nations where children are essentially feral and run wild in the streets, just trying to survive. The kidnappers pose as charities trying to help feed them.

They also come from hidden breeding facilities. They are not seen and, again, there is no record of their births.

You're probably right, but if they are breeding children underground it would be possible.

Many born underground. They are not all part of any AU census.