From the following discussion (http://wpftoolkit.codeplex.com/discussions/252462), it seems it is possible to access the main window using shortcut keys even if the ChildWindow is modal.
Comments: ** Comment from web user: BoucherS **
Comments: ** Comment from web user: BoucherS **
Hi,
This will be fixed in v3.0.
Modal ChildWindow and MessageBox Tab Navigation will remain in the Control. Also, Menu shortcut keys won't be available when a Modal ChildWindow or MessageBox is displayed.