For the strawberry sauce
Combine the strawberries, sugar, vanilla and lemon in a pot. Place the pot over high heat, boil for 5 minutes or until strawberries have started to break down. Mix together the cornstarch with 2 teaspoons cold water. Add the cornstarch mixture to the strawberries and boil for 1 additional minute until sauce thickens slightly; cool.
For the blueberry sauce:
Combine the blueberries, sugar and vanilla in a pot. Place the pot over high heat, boil for 5 minutes or until blueberries have started to break down. Mix together the cornstarch with 1 teaspoon cold water. Add the cornstarch mixture to the blueberries and boil for 1 additional minute until sauce thickens slightly; cool.
For the strawberries:
Melt the candy melts in the microwave according to package instructions. Dip each strawberry into the candy melts and coat in sprinkles. Place on a piece of parchment, let dry until completely hard.
For the watermelon:
Cut lengthwise across the watermelon approximately 1/2 to 2/3 of the way down the watermelon. Cut a very thin slice off the bottom of the watermelon so that it will sit flat. Using a spoon, carve out some of the watermelon flesh, about 1-2 inches deep. Drain off excess juice. Freeze until completely solid (time will depend on the size of your melon).
Pile up scoops of vanilla, strawberry and blueberry ice cream in the watermelon bowl (amount will vary depending on the size of your watermelon).
Top the ice cream with the strawberry, blueberry and marshmallow sauces. Add whipped cream, sprinkles and white chocolate covered strawberries on top. Serve.