Hi Mardukes,
I don't believe any of the 6 relationship behaviours cascade 'deactivation' from a parent to the children. Deletion is probably your closest alternative but obviously, may not be desired!
To acheive this using workflow would not be very elegant as you'd need to have waiting steps on each contact that monitor the status of the parent account.
I would go down the plug-in route to de-activate child contacts when the account is deactivated.
Rob
Microsoft Certified Technology Specialist (CRM) GAP Consulting Ltd.