Open the can of lychees and pour them into a strainer set over a bowl. Keep the syrup, because you will stir some into the punch later. Pat the lychees dry with a paper towel so they freeze cleanly.
Look at each lychee and find the hollow where the seed used to be. Gently push one or two blueberries into that cavity, depending on the size. The dark berry peeking out makes a perfect creepy pupil.
Line a baking sheet with foil and set the stuffed lychees on top, spaced apart. Slide the tray into the freezer on a flat spot. Leave them for about one hour, until they feel firm all the way through.
Pour 5 tablespoons of lychee syrup into a large bowl or pitcher. Add the blueberry juice cocktail and the fresh lime juice. Stir until the color looks even and the syrup has fully blended in.
If you are making the adult version, stir in the gin now. Do this before the sparkling water so the bubbles are not flattened. Make a separate small pitcher if you want kid-friendly and adult options side by side.
Slowly pour the sparkling water down the side of the pitcher. Stir once or twice with a long spoon, nothing more. Gentle stirring keeps this berry eyeball punch bright and bubbly.
Drop the frozen lychee eyeballs into the punch and watch them float. Take a small sip to check the balance. If it tastes too tart, stir in the last tablespoon of lychee syrup.
Fill tall glasses with ice and pour the punch over the top. Use a spoon to add a few eyeballs to each glass. Serve right away while the punch is cold and fizzy.