I would suggest that "imagination" as a mere flight of fancy isn't the only thing to be considered here. I would imagine it took considerable imagination for Einstein to develop the theory of relativity, or for Bach to write the St. Matthew Passion, but neither did so without referring to a lot of structure and knowledge that they already had.
"Quadrupillion" implies four of something, when actually it's a googolplex plus a hundred (This actually is a pretty damn useless number unless the number (googolplex+100) turned out to be very important to some particular idea or occurrence, but that's less important to my point). It would be better to tell the child this, and explain about the quad- prefix, and suggest a better name for his number. As for its uselessness, I imagine that this will become apparent to him quite quickly, so that doesn't matter so much in terms of what to explain to the child.
"Quadrupillion" implies four of something, when actually it's a googolplex plus a hundred (This actually is a pretty damn useless number unless the number (googolplex+100) turned out to be very important to some particular idea or occurrence, but that's less important to my point). It would be better to tell the child this, and explain about the quad- prefix, and suggest a better name for his number. As for its uselessness, I imagine that this will become apparent to him quite quickly, so that doesn't matter so much in terms of what to explain to the child.