Heck no. If Sesame Street can devote an entire episode to the death of one of their main human characters, then there's no reason why cartoons aimed at kids can't either. In fact, I think it's better for kid shows to deal with disease and death rather than whitewashing them - just because cartoons are suppose to be an escape from reality doesn't mean that they have to be entirely unrealistic, and having realisrtic elements is a good way for them to be meaningful rather than having them be accused of being mindless