I'm building a toy theatre out of wood and I want it to be easily disassemblable for packing down. If the front and sides are basically flat planes of wood meeting at an edge:
(top down view, walls in blue, possible attachment points in green)
what kind of quick-release fastening is there which I could screw on either the inside or outside (inside preferred) which would hold these side together reasonable well? They wouldn't have to hold against much torque.
Instead of disassembling the toy theatre, I would attach the sides with (piano)hinges. That way you can fold them in to create a flat package for storage.
Bed rail fasteners would be easy to assemble/disassemble and durable.
Photo from Amazon.
Not a recommendation: amazon linky
The loose pin hinge will probably be your best friend, here. How good you want them to be will likely depend on how often the set will be used, and whether you plan on pulling them off and using them on new sets.
Avoid buying from theatrical supply places, they charge way too much. And/or avoid buying from non-theatrical supply places, their hinges won't be up to snuff.
You can also, perhaps much cheaper, create something similar with eyelets and rebar. Or taking regular hinges and replacing the pins. Or if you work in a school, the people in shop can likely whip something up for you.
As well as tear-down, the hinging can also allow for sets to be folded or rotated to one side, making for a super-fast set change.
A brace on the inner, concave angle can provide some extra rigidity and keep the leaves from folding together while in use. If that side's facing the audience, you can have the brace be a matte black stick across the top, with holes either end to slot onto bolts sticking up from top of the flat.
Generally speaking, though, this is likely over-engineering for a short run, and G-clamps are the way to go.
(via via https://www.gerriets.com/us/triple-e-loose-pin-hinge)
(via via https://www.theatrecrafts.com/pages/home/topics/scenic-stage-design/flats-types-methods/)
You might consider something like these cam latches for the outside attachment point. These things are the epitome of quick-release. 
Cam lock furniture fittings - Ikea style flat pack furniture uses them a lot.
C clamp
Easy peasy. You probably already have some. Once clamped into place draw a red line across all pieces on one side and then a blue line across all pieces on the other side. When you or the kids want to put it back together, assemble the red line pieces and the blue line pieces and clamp each in place.
The clamps will show. That is ok.
When using modular panels for an art trade show, we had the free standing sections attached with a "hook-style concealed panel lock" aka "coffin-lock."
Very sturdy, draws the panels together, uses an allen-wrench key system for assembly/disassembly. Virtually invisible (only the hole for the key is visible). Requires mortise.
Dowels would be a simple method.
Then screws just to lock in place.
