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Parodies of Youtube Kids and Cocomelon would make good KND villains now that i think about it.
An adult villain who mass produces candy-colored, overstimulating videos featuring Rainbow Monkeys and Yipper to get kids addicted and keep them inside. (And they get defeated by a kid who avoids their videos because they give them sensory overloads.)
Or making videos showing Rainbow Monkeys getting tortured to scare kids away from Rainbow Monkeys and the happiness they bring.
Or maybe the videos straight up mind control kids for whatever reason.