Would a dog snatch a child from the arms of a bigger human?
My first thought was that the child was left unattended. I can't see it just running up to someone to grab a child out of their hands for no reason.
I tend to agree. First thought was the kid was not in the arms of his mother. Second thought was this dog was, as some point abused, possibly by kids or adults. The story could be true, but I find it suspect.