Robin Thicke -- I Can't Sing ... But I Can GO TO DISNEYLAND

Robin Thicke is one sick dad.

Robin cancelled his concert Saturday at Orlando's Universal Studios because of voice problems ... but still pulled himself together to hit up Disneyland with his son.

Robin also canceled his Friday show in Atlanta. It's unclear specifically what ails him but whatever it is ... it must not be contagious because he spent most of the day Sunday waiting in long lines.

Disneyland ... where the lines are 95% of the fun.