@sqw makes a good point about checking for practical measures as well.
The passthrough slots are pictured in my Maker Faire thread
The openings are about 3/8" wide, so kid fingers could fit through. My assumption is that the holes you can see inside the casing above the front passthrough are where the flap would attach. You could wedge a piece of thin metal between the flap and honeycomb and make it impossible to push through at all. And a larger chunk of metal/wood on the back side would secure that one as well (though having the whole Forge backed up against a wall also locks that one down nicely. Just relocate when actually using the pass through)