Duckman, though that was not for kids.

Ren and Stimpy and Rocko’s Modern Life both had stuff that was bizarre and only allowed in a kid’s show because it was over their heads.

Batman TAS and Gargoyles both had some heavy shit. The latter also had a guy die by having fire erupt from behind his eyeballs, and a scene where a surgeon explains a gunshot wound in visceral detail. God I love that show.