Let's see, in slow motion, when you hit a bump, the rear wheel will move up first, then it's force thru the shocksprings will move the frame up, the upward moving frame will compress the seat springs, the seat springs will apply upward force onto the seat and rider, the rider will bounce up, the upward moving rider will accelerate relative to the lower jawbone, allowing mouth to open, altering shape of giddy grin typical of XS riders, looking something like this:
Wasn't sure about your rear fender plans, thought you were going to try it fenderless. Didn't want to see the tire chew into your seat, so was gonna suggest maybe a temporary 2-3" skidplate fitted to bottom of seat, then check for skidmarks after test ride...