They see that rash as not that scary, and the rash is honestly the mildest part of the disease. Measles can cause encephalitis (brain swelling) and kill the child, it can cause pneumonia and kill the child, they can recover from the illness and be completely fine for a few years until the virus reactivates and their entire central nervous system becomes intractably inflamed and they seize until they die. And there's nothing we can do about any of those complications besides things like IV fluids or ventilatory support because there are no antiviral medications effective against measles, so we just have to hope the child's immune system wins.