I have a networking degree and in my opinion your options look very limited. I don't think it is physically possible to do this without running power. At least not at 100m.
One option I found is startek makes ethernet extenders for that distance at a big cost. http://www.neweggbusiness.com/bs/startech-extenders-repeaters/id-1762-3029 But you would still be left with needing power or some kind of battery pack. One thing you can look into is power over Ethernet but that is quite difficult to do without a network technician and likely more costs for additional equipment. Even then I am just not sure this would work due to the lengths and usb controllers not designed for that much length. For a device like a mouse or keyboard it would likely work but I wouldn't bet on a high bandwidth device like the leap to work. So if you do go this route make sure you have a good return policy.
You could look into a usb extension but even those aren't designed to go far. The longest I've seen is 40 meters when chaining two of these together http://www.amazon.com/Tripp-Lite-Hi-Speed-Extension-U026-20M/dp/B006KSLF6E . Even then you would need a powered usb hub at the end. While this may work there is still risk involved.
If it were me I'd relocate the pc near the pedestal and do my best to hide it. You could also look into getting a tiny computer that meets the leap's system requirements like this http://www.amazon.com/Asus-VM60-G067R-ASUS-Desktop/dp/B00J2DXUWQ which is easier to hide but still costly depending on your budget. It is much easier to extend hdmi than usb if needed.