Like "Blecha" said, these are all good recommendations. Indeed.
Larry touched on the perils of "the smallest species" of snakes. I definitley don't want to insult your intelligence but I would like to make sure you're aware of the perils of keeping the majority of the smallest spcies.
Specialized prey, specialized living conditions, and, last but not least, handling sensitivities, whether handled by an adult or child, either one. But more so with a child.
From what's been mentioned already I particularly suggest either a sand boa, rosy boa or hognose. Feeding will probably be the deciding factor.
These are docile in nature and aren't so fast moving that makes handling by a child difficult.
I commend you for being willing to take on the responsibility in order for your child to enjoy & learn about snakes. I've always considered it an extra perk to being a father, we get to re-live our childhood days. Thanks to my son for rekindling the passion within me.
Best wishes!
HH
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Due to political correctness run amuck,
this ol' hillbilly is now referred to as an:
Appalachian American