i would go with a rosy boa; they are hefty enough to not instill a fear of them wriggling away/spazzing out, but small enough that even a child can handle them by themselves. they also have tiny little heads and cute faces.
I also think they are neat becuase they are slightly more endemic than the KSBs
so you can talk about adaptations to predators (blunt tail, live underground etc) in N.A., vs africa
9not that KSBs dont have those adaptations too...). that being said, KSBs tend to be a bit more flashy and have lots of cool morphs... Get both? lol
I believe that there is such a thing as too small of a snake for newbies/folks that might be afraid, and i think rosy's fit the spot perfectly