here's the dimensions: opening is about 5ft wide and 27" tall. the floor of the loft is 65" up the wall.

I did a similar removable railing with some L brackets for the top rail, and routed keyhole slots that slid own over a bolt head (out about 1/2") so only two screws held it in place an it then would lift up & off. For completely hidden, you could use the bolt heads and keyhole slots for all 4 points, but it would not be tamper-proof like with screws.
The L brackets were inverted so the parts that screwed into the newel posts were hidden by the rail ends, but the bottom showed, it was mortised into the rail so it was flush and there was a mechanical joint making it firmer. In your case you would mount 1x6's or whatever to the walls, and use them in place of newel posts. If you try to use the drywall, it's going to get torn up with the taking out an putting back.
Casey

This post was edited by sombreuil_mongrel on Mon, Jul 21, 14 at 20:35

Maybe you need something like this, brass threaded inserts, to let you bolt, unbolt, and rebolt the gate (which could be a simple wooden frame) to the wooden "newel posts" that Casey was talking about. (I do agree, you need a stationery wood (or metal) "newel post" to attach the gate to on either side; either w/ a hinge on one end, or with locks on both ends.

http://www.woodcraft.com/category/4/1002228/2001048/Threaded%20Inserts.aspx

threaded brass inserts, that you sink into the newel posts, and then you can put a bolt through the gate frame and into the brass holes over and over again.

The plans that I once was considering for a folding workbench had you make a long bolt w/ a T-shaped handle; You attach a T-knob to a bolt, then slide the bolt through the gate and into a hole drilled into the "newel post."
http://www.rockler.com/2-t-knob

if you could make something like that, it could be your bolt that holds the gate in place. (though the handle might make it too removable, while a countersunk bolt like the ones in the link above could be removed by you but not by the kids)

http://ths.gardenweb.com/forums/load/wood/msg0915304619900.html

I wonder if this sort of hardware would help:

http://www.rockler.com/bed-rail-connection-system

I think this is a similar piece of hardware--"great for knockdown or permanent applications"--you're the "knockdown" person.

http://www.rockler.com/heavy-duty-universal-connector

(another bolt-with-a-knob: http://www.rockler.com/5-star-jig-knobs-through-hole-insert )