Well, that doesn't really seem fair. Her son couldn't believe that someone didn't know about a Fast Pass. I've seen plenty of adults on these boards making fun of strangers for thinking there is a fee to use them or talking about those in the standby line thinking they were cutting when using Fast Pass.
And the OP seemed to want to be able to give her son an actual statistic so she could reinforce the idea to him that not every kid in the U.S. goes to WDW multiple times.
I also think it depends on where you live. Most people I know haven't taken their kids to WDW. Not because they don't like to travel but because they just don't have any interest in going. I tease one of my friends that I'm going to take her dd with us because even though she's 6 and she's been all over the world on vacation, her mom is mean because she won't take her to WDW.

It's not always a matter of not being able to afford it but more of a lack of interest.