I agree that you should do the Pirate's League. My son did this right after he turned three. He received his pirate name (Leonn Bridgekidd), a sword, an eye patch, a necklace, a tattoo, a baggie, his face painted, a bandana, a picture (use the code on it to add to your PhotoPass card). The price has gone down, so it may include less now.
We did the Pirates and Pals Voyage (cruise) right after that. There is no minimum number of people for this, since it is sold until the boat is filled. I did it with just my son and husband. About 45-50 other people will join you. That is an estimate. I did not count.
Before the cruise, you meet at the Contemporary where you get snacks (you can take some for later). Take some pictues with Captain Hook and Smee. Then you board and hear funny stories from your Captain, Patch. When the Magic Kingdom fireworks start, they stop the boat for you to watch. After the cruise, you go back to the dock to meet Peter Pan. The characters are more geared toward boys, but girls would love it too I bet.