Here are my steps to reproduce:
Go to Control Panel.
Click on User Groups.
Add new User Group and Save.
Click on Actions > Manage Site Pages.
Add Page and Save.
Add Childpage and Save.
Go back to User Groups.
Click on Actions > Go to the Site's Private Pages and check it - everything is ok.
Go back and click on Actions > Assign Members and assign one user to the user group.
Open User's Public Pages - Childpage occour's in Toplevel-Navigation - that's not ok.