I made a secret door a couple of months ago. Yours is way nicer than mine though! (Mine was for a utility room, to keep the kid out of the room with the sharp things and cleaning supplies)
The video @wtnhighlander posted is what inspired me to give it a shot. I went with a much simpler latch though. My doorframe had a plank at the top, which I considered the top of my cabinet. On the top of said cabinet, I will (yeah, haven't latched it quite yet) put a dowel sticking straight up. Then I'll put some sort of decorative sign on the face plate, that will actually punch through the top plank with a hook. Move the sign to one side, it unlocks the door. Move it the other way, and the hook will catch on the dowel, holding it closed
If you actually want your kid getting through while young though...this might not be viable. However, you might use that low ceiling as the spot where your magnets mount!
Probably off topic, but would a cyanoacrylate (instant/super glue) work as well to fill these cracks as epoxy? I have a somewhat similar (slab with cracks) project coming up, so I'll be interested to know how your approach works.