I think with children it can make sense sometimes. Inserting ideas that a child can’t understand, like violence (mortality) and sex is likely to confuse them, and even adults tend to be upset about confusion. Of course they will still probably always be exposed to these things to some degree, but it’s better to set them up to take these topics seriously, because it is certainly the case that many adults do and do not take them seriously, and whether they joke about them or not.
You could maybe make the same about mentally handicapped adults, but if they’re already going to succeed as an adult, they have presumably learned what they need to know about what is appropriate or inappropriate to say to other adults (for the most part).