While many remember the ’90s and early ’00s as a time filled with grunge music, body glitter and the World Wide Web, those who grew up in the era believe the real icons were NickToons and slime.

Some parents look at their toddlers and see bottles, bibs and strained peas. Gabor Csupo and Arlene Klasky – husband-and-wife animators – saw something quite different: a cartoon. And what a cartoon ...