It’s Talking Math with Your Kids week here on OMT.
We’ll get started with a favorite topic: large numbers.
My son Griffin was thinking about large numbers in the car the other day. He was trying to figure out what good it is to have a number (here, googolplex, which for the record is 
Griffin: If you put all the things [in the world] together, would that make googolplex?
Me: No.
G: Even if it’s nanoinches?
Me: Nope. Still not googolplex.
G: Even if it’s half-nanoinches?
G: Even if it’s all of the seconds of the world being alive?
Me: Nope.
G: Even if all the seconds of the universe existing?
Me: No.
I love the developing proportional reasoning embedded in Griffin’s questions.
For each example, he scales it up when his first try doesn’t do it.
If nanoinches don’t work, surely half-nanoninches will! Plenty still to learn about orders of magnitude, I’m afraid.
Overthinking my teaching 
Even though Griffin might have plenty more to learn about orders of magnitude, this could easily be a discussion I have with my high school kids, and Griffin would be holding is own. For starters, he realizes that dividing by 1/2 is the same as multiplication by 2, which is nontrivial, even in 11th grade. Second, he realizes that doubling things does make them grow quickly, so it’s not a bad strategy. In fact, it won’t take him any more then about 3 doublings to bump up an order of magnitude (since
). He can hardly be blamed for having a tough time grasping how many doublings would be involved in the case of a googolplex. Third, this question of “What good is it to have a number that…?” is rampant in my curriculum as we move through, e.g., irrational and imaginary/complex numbers. So yeah, Griffin’s a stud. I want to teach your kids some day.
Here’s something that might foster further discussion.
I have this kind of discussion all the time. I like the current estimation that all the particles in the universe add up to ~10^80 and so even just googol is not very useful from a counting perspective.