C.K.
"Normal" sleep varies for kids just like it does for adults. I know some adults who feel refreshed after 6 hours, while others need 9, a difference of 50%.
If your kids are not cranky or sleepy during the day, they are fine. (I'd tell your coworker to mind her own business!) Her kids need more sleep and that's cool, but YOU are the best judge of how much sleep your kids need--not your coworker or a book.