My 21-month old son is extremely gentle with all of my ball pythons but I would never let him hold them until he's a lot older. But I've taught him to pet them very gently and he loves to touch them. He's very intrigued by them and loves watching them too. He also has never shown the least bit of aggression.
These are older kids and I'm quite sure could handle holding a little snake for a minute or two but you also said they are mentally handicapped. What age are they mentally? Have you met them, and after being around them do you believe they are capable of handling the snake? If you have the slightest bit of doubt, don't let them, instead just let them pet it. If you told them how to handle them, would they understand it? Is the snake aggressive in the least bit? You said it is only 20 inches, it's probably still a baby and most babies can be very defensive.