Like the other areas in previous episodes, you get a maximum of three stars in each area, one for completing the story in the area, one for doing certain actions on a set of 5 items in the area and one for finding all the lost items.
The Pixie Hollow Games episode has three areas instead of five. There are five fireworks to set off and six lost items to find in each area. If you complete the three stories, set off all 15 fireworks and find all 18 lost items, you will get your 9 stars (3 per area) needed to get this.
Remember, all the fireworks are located in the main sections of each area. However, the lost items can be found in the main sections and nooks. Unlike the other four episodes, lost items are a bit more difficult to find (especially if they are hidden in the main section) in this episode. Try looking in the back areas in the main sections of the areas. A few items are found there. Also, you can look at the list of the lost items for this episode in the main menu to get a better idea of what you have to look for.