I don't think keeping charaters simple is the issue. Issue with OK KO is stylization, especially characters vs. backgrounds. The original Powerpuff Girls, also a comedy action cartoon for kids, holds up really well and those girls are super easy to draw. In PPG everything is made so that the girls stand out and action is easy to read. The girls are made from curves and circles, where as other characters are very pointy with few to none circles. Only the characters and important items have black out outlines, everything that is meant to be just background is in solid colors or have softer contrast with colored lines. Character have stronger, more saturated and higher contrasting colors than the backgrounds.