The expert sidekick has the helpful ability (at level 7+):
The expert can take the Help action as a bonus action, and the creature who receives the help gains a 1d6 bonus to the d20 roll. If that roll is an attack roll, the creature can forgo adding the bonus to it, and then if the attack hits, the creature can add the bonus to the attack’s damage roll against one target.
Since Magic Missile automatically hits and is ruled over the years that one must throw only 1d4 and apply that same roll to each dart, does the damage for each dart then become 1d4 + 1 + 1d6?