If you make it to the parks about 20 minutes before the park opens, then you will be at the front (or close enough) of the line to get into the park. Once they do the rope drop, follow the TM's and the rest of the crowd as they walk you back to HP. Once you get into HP, ride FJ first. There will already be a line becuase of the early entry, but it should not be overly insane like it can get once the park has been open for an hour. Once you are done with FJ, then grab your butterbeer!
There are four places I know of to get Butterbeer. They have two carts with it. One is near the main entrance to HP and the other is near the Jurrasic Park entrance to HP (also close toFJ). Another one is inside the 3 Broomsticks. Finally my personal favorite since it never seems to have a line is inside the Hogs Head Pub. Riding FJ first thing and getting your Butterbeer right after will start off the birthday without much hassle. Enjoy your trip! Only 3 more days for us!!