"...whether or not they would have tea". Use "whether ...or" for two specific options. Use "whether or not" for the choice between a) doing something and b) not doing the same thing.

Thank you Richard I liked your comment but I can’t send you $6,000,000 as I am just a poor student.

Michael's answer is perfect. We can use 'whether or not' in two ways. 1... whether or not they would have tea, and whether they would have tea or not ('have tea' is implied/understood). Just as an extra bit of information, 'if' is used instead of whether sometimes, but only if there is one alternative/choice/condition, e.g. You may send me $6,000,000 if you like my comment. [I wonder whether Timur will send me the money or not... :)