Unless the child is anemic, it's unlikely for anybody, including a child, to be bruised by a snake doing regular snake things, due to the surface area the snake covers. Bruises are generally a more acute impact. Not always, sure, but unless the child was coming in with his entire portions of his body bruised, or broken ribs, I'm not buying it.

We don't know what the deal is with the parents, obviously... but considering this is a story told to you by your mom, or your mom's teacher friend, about a child that came in at some unspecified time, I'm guessing you don't actually know if the child is still coming in with bruises, or whether the story was just somebody's embellishment to the hype surrounding a family's pet snake. Sure the bruises didn't look like an adult's hand gripped firmly around the child's arm or legs? Sides? Any bruises on the face, i'd be even more suspicious about the parents.

The manner in which snakes move, and hold onto branches, is unlikely to bruise a human during regular interaction, unless, as I stated, the child is anemic or has some other brusiing/bleeding disorder. I'm not saying it's impossible for them to bruise.. very large constrictors are obviously capable of much worse than bruises.. but I'd be surprised to see the types of bruises that would result, on a small child, and have that child still be alive, if in fact this were a murderous snake that strikes and bites and constricts as frequently as this child's bruises would suggest.