Time scales are particularly hard for kids
Try showing a light second, and building up.
"In one second light goes 300K klicks..." and focus on the scale of just how huge 300k klicks is.
Maybe line up the boys and ask about the distance of famioiar landmarks they have maybe hiked to or visited or ridden bikes to that are about 10kms. Then aggregate that distance to represent 300, then 3000, then 30000, etc.
Line up the boys at distances scaled to represent part of the demonstration. That may give a sense of the vastness invovled.
Sounds like you are going to have a lot of fun.
Good luck,
Ed