This is outdated help documentation, indeed.
Thank you for reporting this to our team.
I will log it for an update and such will be provided soon after the upcoming release (Q2 2011).
The CheckChildNodes is always true when Tri-State Checkboxes are enabled.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!