So days later, a jungle gym has emerged from my 3D printer. I had to learn a PC program called 3ds Max from Autodesk, a program I have always wanted to try. It is a high-end 3D design program that I am a mere amateur at, but I did get a printable jungle gym design from hours of fiddling with it.

3Drender
This is what one version of the model looks like in 3ds Max.
lasttry
I gave the drilled wood one last try, using the 3D design as a guide. But I still ended up with broken dowels.
Printing took 9.5 hours and had to be watched the whole time.
Printing took 9.5 hours and had to be watched the whole time.
The result: a bit fragile but should work. Needs to be painted next.
The result: a bit fragile but should work. Needs to be painted next.