Our daughters will be similar ages when we go to WDW. We have never been and I am still in the research stages myself.
I agree with PPs that at those ages, asking them for input/an opinion/preferences would be my plan in your position.
Personally, I am not planning to do either. I am making my daughters Cinderella dresses and bringing them with us as a surprise for a visit to dine in Cinderella's castle. The BBB is frankly too expensive for my sensibilities, despite the fact that we might or might not be able to afford it, IYKWIM. I get that it is an experience, but to me (and I do not presume to speak for others) I would rather that money go toward the meal in the castle. Dresses purchased outside the park (even from the
Disney store online) are cheaper, but then, I clearly don't see enough in the experience to justify the price.
I love high tea, but the one at the GF has only mediocre reviews. I would rather make high tea at an upmarket venue with spectacular food something I do with them at another time and focus on the things that can't be done elsewhere outside WDW.
This, of course, is only my opinion and I am giving it to give you a perspective. You know your kids, your budget and priorities for WDW best, and I am sure you will come to a decision you are happy with. I may even end up changing my mind, depending on what my DDs really want to do. Time will tell!
Have a great time, no matter what you decide to do. It will be a precious experience.