Hi diminos. As you've booked the Family room already, I wouldn't check in yet. Sometimes the upgrades, if any, are processed through the night before arrival. If that doesn't happen, then I would wait until check in and ask politely if there is any possibility of a Diamond upgrade. Worst they can say is no and you get the Family room which you've already booked. You may be lucky. Good luck!! Also in my experience, UK Hiltons never charge you for restaurant breakfast for a child staying in the room with parents with Diamond status. I have often booked 2 rooms and been given breakfast for 4 guests.
Hi Cjd
Thank you for this and i will follow your suggestion and update here accordingly.