In medium microwave-safe bowl, combine thawed lemonade concentrate, lemon juice, and sugar. Sprinkle gelatin over surface of liquid. Use rubber spatula to stir until no large lumps remain. Let mixture sit for 5 minutes.
Heat lemonade mixture in microwave for 1 minute. Stir mixture with rubber spatula. Heat in microwave until mixture bubbles and foams, about 1 more minute.
Use oven mitts to remove bowl from microwave (ask an adult for help—mixture will be VERY hot). Stir mixture until fully combined and smooth.
Place 2 silicone lemon slice molds on rimmed baking sheet. Use eyedropper to fill molds following photos, 'Step-by-Step: How to Fill Silicone Molds,' below.
Place baking sheet with filled molds in refrigerator. Chill until lemonade mixture is firm, about 30 minutes.
Remove baking sheet with molds from refrigerator. Push each shape inside out to remove fruit snacks, and transfer to airtight container.
Heat remaining lemonade mixture in microwave until warmed through and liquid again, about 30 seconds. Repeat steps 4 through 6 using clean eyedropper and molds and remaining warm lemonade mixture. Serve. (Fruit snacks can be refrigerated in airtight container for up to 1 week.)
