I was surprised to find that the thought of a tea party held no appeal for the adults I talked to. Maybe that's because the Disney Princess people took over the idea. These days a lot of formal kid birthday parties are built around this Disney theme. I guess it's a chance for girls to wear the princess costumes they bought at Halloween. There's nothing wrong with that, but I prefer something on a smaller scale....
...something like this (above). These kids know how to celebrate.
How old should the kid host be? Maybe as young as this girl (above). Boy, she really has the tea party spirit. I like the small scale of the furniture and the tiny plastic tea sets. The chair should be so low that an adult sitting in it has his knees almost up around his chin.
Of course a tea party requires ritual. Maybe a little kid has to have witnessed an adult tea party in order understand the ceremony.
Kids have a limited attention span, and the tea party probably won't last long. Maybe you can stretch it out by baking scones with the kid, following her directions. Scones are fast and easy to make.
Maybe the thing to do is to build up expectation for the eventual arrival of...The Queen. Maybe the Queen is an adult who comes in amid much fanfare and agrees to take tea with everyone for a while. Here's (above) the way the Rennaisance Faire handles the Queen's entrance.