Add the blackberries to a medium bowl and sprinkle the sugar over in an even layer. Mix them together and mash with a fork. Let the mixture sit for 20 minutes. Add any extra juices from the watermelon, up to 2 tablespoons, to the berry mixture and stir to combine.
To serve, arrange 1 cup of the watermelon cubes into an 'X' shape in a banana split dish. Place one scoop of the ice cream in the middle. Scatter 2 tablespoons of the blackberry sauce over the ice cream. Top with a dollop of whipped cream, 1 or 2 cherries and a tablespoon of nuts. Repeat for the remaining servings.
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Kids Warning: Note to parents: Never leave a child unattended in the kitchen. These recipes are appropriate for children of various ages, 4 to 10. When a recipe calls for cooking on the stove or using a paring knife, an adult should do those activities and let the child assist, if age appropriate.