Four-year-old gwen and eight-year-old stefani are coming back from their grandfather's funeral. which of them understands that eventually everyone has to die?

The right answer is Stefani. Children understand the concept of death differently according to age. Between 2 and 4 years they see death as a reversible and temporary process. Between 5 and 9 years old they understand death as something definitive and a process by which all people must go through. Finally, at age 10 they have a complete concept of death.
 
I hope my answer can help you.