The aim was over time to combine them into there parent threads.
So, should we avoid posting any new threads into the "child board" forums?
And just write everything into the parent forums?
If that is the goal, then you might add a sticky note to each child board that they are in a retirement mode and that new threads should be posted into the main forums.
No continue to post in the child boards.
Its quite easy for me to move the posts across.
If for example some are more active than others than they will become a parent board.
Mini-itx is one that I expect will do this.