"Infinity is basically forever."
"It's not a number, and it never ends."
"It's technically the biggest number, but it's not a number."
"If it is turned sideways, it is an 8."
So you can see they came in with some rudimentary knowledge about a very complex concept. Children absolutely love to think about "big" things - pondering the enormity of the universe, or wondering life millions of years ago.