Here's my setup:
From the User application: the user has a Privilege level of "Editor", and is in the role "Role A".
From the Permissions application: My site is selected in the first dropdown; "Page Type" is selected, and "Event (booking system)" is selected. "Role A" has "Create" permissions checked.
From the Pages application: Using the Security tab of my "Calendar" page, "Role A" has "Create" permissions (I've also tried giving the user Create permissions directly, with the same result).
Now, when the user tries to add an Event (booking system) under the Calendar page, it fails with the error I posted earlier ("The current page cannot contain any child pages..."). However, if I go to the Permissions application and give "Role A" the "Create Anywhere" permission, the user is then able to add an event under the Calendar. The problem is, I don't want them to have Create Anywhere permissions -- I want them limited to just creating events under my "Calendar" page.
Let me know if there's more specific information I can give.