I have booked this offer several times for my family and CJD is correct it is hotel specific and you have to telephone customer care who telephone the hotel directly to see if they can offer it.
The 50% rate is off the flexible rate only and not any discounted or sale rates.
I recommend you also confirm breakfast will be extended to the second room for your children and lounge access, I have always checked this prior to booking and it has always been honoured but if you do not pre confirm you may be hit for high breakfast charges on your kids room making it no cheaper.