How I've been running my games is generally in agreement with marc's second definition in which the only things the enemies know is the attack/spell and its immediate effects from the list of effects/conditions in the PHB.
And the reason why I think monsters know about divine challenge damage...
well...
On page PHB 278, it says:
Maximum Damage: Rather than roll damage, determine the maximum damage you can roll with your attack.
And PHB 117:
Sneak Attack
[...] You decide whether to apply the extra damage after making the damage roll.
So the sneak attack getting the crit...