My nine-year-old has dined there a few times, (we are local) but I've never had to go with her (birthday parties and grandparents). I will warn you, though, a big appeal of it is that there are special seats for your doll, plus special tiny dining items. They even have places for your doll in the bathrooms! So, if she has a doll and you can bring it without too much trouble, do it. If you are going to buy a doll, buy it before the dining experience. I can't speak to the tea vs. dining. I don't think it's so much about the food, but the experience, so if you can get away with just tea, I'd totally do it. (But then, if you're just going to have to pay for lunch and dinner somewhere else, maybe it's better to just eat there?)
It depends on what time of year you're going, but yes, I definitely recommend reservations. That place can get packed. Have fun!