This is my version of a 'fort building kit' for my kids.
Simply take 1/2 inch PVC pipe cut into convenient sizes (I use 2 foot & 1 foot), print out the appropriate connectors, and have fun! I even include clips to hold sheets or thin blankets to the frame and a couple jigs to help you cut the PVC straight (both a manual cutting jig and one for use with a power tool such as a scroll saw).
All the connectors use minimal filament and are designed for easy printing (no serious overhangs). You might need to play with the scaling as you want the pieces to be tight (not loose) but not too tight. I'd recommend printing the pieces with 3 top & bottom layers and 3 perimeters as you want the pieces strong so they can withstand abuse by kids.
UPDATE 1: I added a convenient inventory document for keeping track of what you have. This works well in storage box to hold all the pieces together (see picture).
UPDATE 2: I added a 6-way piece, at the request of my daughter. This is more challenging and requires decent bridging ability & some superglue to assemble. I also updated the inventory document.