The Dons coasted to a fifth win in six games thanks to Chadwick's brace and further goals from Shaun Williams, Jon Otsemobor and substitute Ryan Lowe at stadium:mk.
Substitute Gavin Massey was sent off after 67 minutes for the U's, who only had Ian Henderson's consolation goal to show for their efforts.
The hosts controlled the first half and Chadwick put them ahead after 28 minutes when he latched onto debutant Patrick Bamford's clever pass and slotted the ball past goalkeeper John Sullivan.
Williams doubled their lead nine minutes later, sending Sullivan the wrong way from the penalty spot after Bamford had been tripped by Tom Eastman. Otsemobor made it 3-0 early in the second half when he ghosted in unmarked to head home Dean Bowditch's corner.
Henderson gave Colchester a slither of hope when he seized upon some slack Dons defending to fire past David Martin from 12 yards. But Chadwick made it 4-1 when he intercepted Eastman's poor backpass and beat Sullivan.
Massey was then shown a straight red card for a tackle on Adam Chicksen before Lowe curled a sublime 22-yard shot into the top corner of the net to seal the rout late on.