It is frustrating, especially when it interferes with your ability to be able to plan your day. You're expected to know what you want to do and where you want to eat at 180 days out, and it seems like Disney should have their plans set by that date on their end, too. All too often, they're not.
That said, you don't want to miss out on something YOU want to do just because they're being uncooperative. I would keep trying for the tea party periodically. Since no one can book right now, it's not in any danger of being sold out. We had this happen with Pirates and Pals Fireworks Cruise earlier this year -- for some reason, it just wasn't loaded in the system for weeks after the 180 day date. I called a couple of times a week, and eventually I could reserve it.
As a backup, you could book the GF tea. We did it after my daughter did BBB when she was 7, and she loved it. They have a special kids' tea with chocolate milk, and they'll probably treat her in a very grown-up (but sweet) manner, which DD loved. There's either a pianist or a band playing, so there's a lot of atmosphere. And, if you want to, you could even order a special dessert for her: either a chocolate Cinderella slipper with mousse (only $15 and very cute), or you could even do a regular or custom cake to have delivered.