They first and only time they've appeared in the series, was in "Summer Camp" in a flashback when they threw Dan out at Camp Atrocious, only the hands of one of Dan's parents were seen, it's unknown which one.
It's implied or outright shown that Dan has poor relationships with his Mother and Father. Dan's parents are divorced, possibly due to their negative ways of child rearing, either by one or both adults.
In "Anger Management" when Dan was instructed to hit a red button every time something was shown on screen that made him angry, he aggressively clicked the red button at the word "Mother".
Dan at one point made his parents so angry, that they dropped him off at one of the worst summer camps, Camp Atrocious, and didn't even let him have any say.