What do you conceive as "danger" ?
Death? or Injury? A bite?
Death is very remote. You would literally have a large Ball Python, drape it around the kids neck, and then get it to constrict the neck and squeeze hard and long enough. Very remote indeed, but no-one can tell you that would never happen.
If anything, a bite. But they wouldn't die from that. It might hurt a bit, depending how big the snake is, whether its a defending nip or a "feeding error" bite.
Then, like its been said, is the remote danger of salmonella. If the kids wash their hands after handling the snake or cleaning the enclosure (if they ever do that) that is also nothing to worry about.
I find rodents (even hamsters), cats, dogs, horses much more of an "issue" then Ball Pythons, when it comes to possibly injury to children.