I'm not saying it's useless to take kids to startup offices -- it socializes the value of entrepreneurship -- but it's not quite the same experience as seeing the size of an african elephant for the first time, or looking a gorllia in the eye.
* I use "guys" in a gender-neutral sense here; I don't know what the gender ratio was at Google at 10 employees.
In fairness, most parents have probably taken their kids to the zoo dozens of times before they even get to school age. I know I will, when I have kids, because... hey, zoos are awesome!
And heck, even if you had, would it be that life-changing? It'd be a moderately interesting cocktail party anecdote at best. And I say that as someone who is interested in this kind of stuff. For everyone else it'd probably be a fairly boring cocktail party anecdote.
Visiting a couple of guys in a small office really isn't that intrinsically exciting.