Hi, Catherine!
I completely understand your concern; 7:30 is late for dinner when your little Princess is only 3.
You have a few ways to work with this, so hang in there! First, keep going back to
Walt Disney World's online dining reservation system or call (407) WDW-DINE as your visit gets closer. There will surely be cancellations as Guests change their plans; you might be able to get one of those reservations as soon as they become available.
Next, consider dining at
Be Our Guest Restaurant at lunchtime, when the restaurant is a Quick Service location. You won't need a reservation, and may not even have to wait to get in if you get to the restaurant as as soon as it opens for lunch. You'll be able to choose the room you'd like to dine in and enjoy the space just the same as if you were dining at dinner. The menu is different, but some of the desserts are exactly the same.
Finally, if you opt to stick with your original reservation, see if a nap and a late meal will help your little one sit through a 7:30 dinner. The space is very interesting to watch, and, if she's not over-tired or hungry, perhaps some coloring pages and little figures (a must when we dined out with a 3-year-old) would help to occupy the time as well. Tell your server upon being seated that you'd like to expedite dinner if at all possible, since the little one may be tired; that way no one will try to hold your entree because you haven't yet finished your appetizer.
Again, keep trying to get an earlier reservation, Catherine; if you do, problem solved!
Send more questions as you plan your first trip to WDW, too. We love to help!