Hi Sharon,
It's always lovely to be able to answer a question from a fellow Brit!
I think that
Afternoon Tea at the Garden View Tea Room at Disney's Grand Floridian Resort & Spa sounds like a practically perfect way to celebrate your Mums 70th birthday.
I'm not entirely sure that I am reading your question correctly but I think that you are wanting to know if the rest of your family, who will be joining you and your Mum for afternoon tea, will have to have tea to drink?
According to the menu on the unofficial website AllEars.net (http://allears.net/dining/menu/garden-view-tea-room/afternoon-tea) tea is included as a drink for all of the various options available. There are however many other beverages listed as being available (for an additional charge) at this dining location.
If I have not understood your question correctly, please do not hesitate to visit us again at the Disney Parks Moms Panel website and ask me another question.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma