saw an open source project for 3d printing prosthetic limbs with a focus on making affordable prosthetics for kids since they grow so quickly they need new fittings quickly as well.

Unfortunately 3d printing has mainly been a bit of a gimmick in the field of prosthetics, especially the more diy projects. Most people think that prosthetics is an engineering field with a side of medicine, when in reality it's more of a medical field with a side of engineering.

The project you were referring to never really took off because it ended up being detrimental to the patient's long-term health. With how quickly children adapt to their conditions, if you don't provide them with a prosthetic that provides more utility than their residual limb, they end up adapting to never wearing any prosthetic. Which in turn can vastly lower their mobility and ability to interact with their environments.

The fact that much of our prosthetics technology isn't that different from what they had in the Civil War is sad.

I wouldn't say it's quite that bad. I mean I did carve a wooden socket in school, but haven't ever seen one in a clinic setting. Prosthetic tech really advanced in the 90s with the introduction of materials like carbon fiber, titanium, new thermoplastics, and advanced mechanical knee units. With the amount of repetitive ground force reaction a human body can produce in motions, our field is pretty limited by the advancement of material science.