I have seen a full grown coyote charge, at least move quickly towards, a child, but never an adult. I have had coyotes growl / snarl at me if I interrupted them in something, but they still backed away as they did it.
With that said, while I would not expect a coyote to charge an adult, let alone an adult and two half grown kids, there is no accounting for a single animals actions. The fact that the animal snarled at them without moving away significantly in the process indicates something was off there. If it really did drop dead at his feet, or even within 10 yards of the group, then that was very unusual activity for the animal. Even without looking at the actions of the person, if the basic information presented with regards to the animals actions is anything near correct then it was outside the norm and judging based on the norm goes out the window.
T!